vdpa/mlx5: set_features should allow reset to zero

Commit Message

Si-Wei Liu Feb. 19, 2021, 11:54 a.m. UTC

Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
for legacy") made an exception for legacy guests to reset
features to 0, when config space is accessed before features
are set. We should relieve the verify_min_features() check
and allow features reset to 0 for this case.

It's worth noting that not just legacy guests could access
config space before features are set. For instance, when
feature VIRTIO_NET_F_MTU is advertised some modern driver
will try to access and validate the MTU present in the config
space before virtio features are set. Rejecting reset to 0
prematurely causes correct MTU and link status unable to load
for the very first config space access, rendering issues like
guest showing inaccurate MTU value, or failure to reject
out-of-range MTU.

Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Si-Wei Liu <[e-mail]>
---
 drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
 1 file changed, 1 insertion(+), 14 deletions(-)

Comments

Eli Cohen Feb. 21, 2021, 2:44 p.m. UTC | #1

On Fri, Feb 19, 2021 at 06:54:58AM -0500, Si-Wei Liu wrote:
> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> for legacy") made an exception for legacy guests to reset
> features to 0, when config space is accessed before features
> are set. We should relieve the verify_min_features() check
> and allow features reset to 0 for this case.
> 
> It's worth noting that not just legacy guests could access
> config space before features are set. For instance, when
> feature VIRTIO_NET_F_MTU is advertised some modern driver
> will try to access and validate the MTU present in the config
> space before virtio features are set. Rejecting reset to 0
> prematurely causes correct MTU and link status unable to load
> for the very first config space access, rendering issues like
> guest showing inaccurate MTU value, or failure to reject
> out-of-range MTU.
> 
> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> Signed-off-by: Si-Wei Liu <[e-mail]>
> ---
>  drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>  1 file changed, 1 insertion(+), 14 deletions(-)
> 
> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> index 7c1f789..540dd67 100644
> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
>  	return mvdev->mlx_features;
>  }
>  
> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
> -{
> -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> -		return -EOPNOTSUPP;
> -
> -	return 0;
> -}
> -

But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
support such cases.

Maybe we should call verify_min_features() from mlx5_vdpa_set_status()
just before attempting to call setup_driver().

>  static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>  {
>  	int err;
> @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
>  {
>  	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>  	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> -	int err;
>  
>  	print_features(mvdev, features, true);
>  
> -	err = verify_min_features(mvdev, features);
> -	if (err)
> -		return err;
> -
>  	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
>  	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>  	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
> -	return err;
> +	return 0;
>  }
>  
>  static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)
> -- 
> 1.8.3.1
>

Michael S. Tsirkin Feb. 21, 2021, 9:52 p.m. UTC | #2

On Sun, Feb 21, 2021 at 04:44:37PM +0200, Eli Cohen wrote:
> On Fri, Feb 19, 2021 at 06:54:58AM -0500, Si-Wei Liu wrote:
> > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > for legacy") made an exception for legacy guests to reset
> > features to 0, when config space is accessed before features
> > are set. We should relieve the verify_min_features() check
> > and allow features reset to 0 for this case.
> > 
> > It's worth noting that not just legacy guests could access
> > config space before features are set. For instance, when
> > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > will try to access and validate the MTU present in the config
> > space before virtio features are set. Rejecting reset to 0
> > prematurely causes correct MTU and link status unable to load
> > for the very first config space access, rendering issues like
> > guest showing inaccurate MTU value, or failure to reject
> > out-of-range MTU.
> > 
> > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > Signed-off-by: Si-Wei Liu <[e-mail]>
> > ---
> >  drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> >  1 file changed, 1 insertion(+), 14 deletions(-)
> > 
> > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > index 7c1f789..540dd67 100644
> > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
> >  	return mvdev->mlx_features;
> >  }
> >  
> > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
> > -{
> > -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > -		return -EOPNOTSUPP;
> > -
> > -	return 0;
> > -}
> > -
> 
> But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
> support such cases.

Did you mean "catch such cases" rather than "support"?


> Maybe we should call verify_min_features() from mlx5_vdpa_set_status()
> just before attempting to call setup_driver().
> 
> >  static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> >  {
> >  	int err;
> > @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
> >  {
> >  	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> >  	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > -	int err;
> >  
> >  	print_features(mvdev, features, true);
> >  
> > -	err = verify_min_features(mvdev, features);
> > -	if (err)
> > -		return err;
> > -
> >  	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
> >  	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> >  	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
> > -	return err;
> > +	return 0;
> >  }
> >  
> >  static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)
> > -- 
> > 1.8.3.1
> >

Jason Wang Feb. 22, 2021, 4:14 a.m. UTC | #3

On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> for legacy") made an exception for legacy guests to reset
> features to 0, when config space is accessed before features
> are set. We should relieve the verify_min_features() check
> and allow features reset to 0 for this case.
>
> It's worth noting that not just legacy guests could access
> config space before features are set. For instance, when
> feature VIRTIO_NET_F_MTU is advertised some modern driver
> will try to access and validate the MTU present in the config
> space before virtio features are set.


This looks like a spec violation:

"

The following driver-read-only field, mtu only exists if 
VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the 
driver to use.
"

Do we really want to workaround this?

Thanks


> Rejecting reset to 0
> prematurely causes correct MTU and link status unable to load
> for the very first config space access, rendering issues like
> guest showing inaccurate MTU value, or failure to reject
> out-of-range MTU.
>
> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> Signed-off-by: Si-Wei Liu <[e-mail]>
> ---
>   drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>   1 file changed, 1 insertion(+), 14 deletions(-)
>
> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> index 7c1f789..540dd67 100644
> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
>   	return mvdev->mlx_features;
>   }
>   
> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
> -{
> -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> -		return -EOPNOTSUPP;
> -
> -	return 0;
> -}
> -
>   static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>   {
>   	int err;
> @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
>   {
>   	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>   	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> -	int err;
>   
>   	print_features(mvdev, features, true);
>   
> -	err = verify_min_features(mvdev, features);
> -	if (err)
> -		return err;
> -
>   	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
>   	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>   	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
> -	return err;
> +	return 0;
>   }
>   
>   static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)

Eli Cohen Feb. 22, 2021, 6:05 a.m. UTC | #4

On Sun, Feb 21, 2021 at 04:52:05PM -0500, Michael S. Tsirkin wrote:
> On Sun, Feb 21, 2021 at 04:44:37PM +0200, Eli Cohen wrote:
> > On Fri, Feb 19, 2021 at 06:54:58AM -0500, Si-Wei Liu wrote:
> > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > for legacy") made an exception for legacy guests to reset
> > > features to 0, when config space is accessed before features
> > > are set. We should relieve the verify_min_features() check
> > > and allow features reset to 0 for this case.
> > > 
> > > It's worth noting that not just legacy guests could access
> > > config space before features are set. For instance, when
> > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > will try to access and validate the MTU present in the config
> > > space before virtio features are set. Rejecting reset to 0
> > > prematurely causes correct MTU and link status unable to load
> > > for the very first config space access, rendering issues like
> > > guest showing inaccurate MTU value, or failure to reject
> > > out-of-range MTU.
> > > 
> > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > ---
> > >  drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > >  1 file changed, 1 insertion(+), 14 deletions(-)
> > > 
> > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > index 7c1f789..540dd67 100644
> > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > >  	return mvdev->mlx_features;
> > >  }
> > >  
> > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
> > > -{
> > > -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > -		return -EOPNOTSUPP;
> > > -
> > > -	return 0;
> > > -}
> > > -
> > 
> > But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
> > support such cases.
> 
> Did you mean "catch such cases" rather than "support"?
> 

Actually I meant this driver/device does not support such cases.

> 
> > Maybe we should call verify_min_features() from mlx5_vdpa_set_status()
> > just before attempting to call setup_driver().
> > 
> > >  static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > >  {
> > >  	int err;
> > > @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
> > >  {
> > >  	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > >  	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > -	int err;
> > >  
> > >  	print_features(mvdev, features, true);
> > >  
> > > -	err = verify_min_features(mvdev, features);
> > > -	if (err)
> > > -		return err;
> > > -
> > >  	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
> > >  	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > >  	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
> > > -	return err;
> > > +	return 0;
> > >  }
> > >  
> > >  static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)
> > > -- 
> > > 1.8.3.1
> > > 
>

Michael S. Tsirkin Feb. 22, 2021, 7:34 a.m. UTC | #5

On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> 
> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > for legacy") made an exception for legacy guests to reset
> > features to 0, when config space is accessed before features
> > are set. We should relieve the verify_min_features() check
> > and allow features reset to 0 for this case.
> > 
> > It's worth noting that not just legacy guests could access
> > config space before features are set. For instance, when
> > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > will try to access and validate the MTU present in the config
> > space before virtio features are set.
> 
> 
> This looks like a spec violation:
> 
> "
> 
> The following driver-read-only field, mtu only exists if VIRTIO_NET_F_MTU is
> set.
> This field specifies the maximum MTU for the driver to use.
> "
> 
> Do we really want to workaround this?
> 
> Thanks

And also:

The driver MUST follow this sequence to initialize a device:
1. Reset the device.
2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
3. Set the DRIVER status bit: the guest OS knows how to drive the device.
4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
fields to check that it can support the device before accepting it.
5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
support our subset of features and the device is unusable.
7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
8. Set the DRIVER_OK status bit. At this point the device is “live”.


so accessing config space before FEATURES_OK is a spec violation, right?


> 
> > Rejecting reset to 0
> > prematurely causes correct MTU and link status unable to load
> > for the very first config space access, rendering issues like
> > guest showing inaccurate MTU value, or failure to reject
> > out-of-range MTU.
> > 
> > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > Signed-off-by: Si-Wei Liu <[e-mail]>
> > ---
> >   drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> >   1 file changed, 1 insertion(+), 14 deletions(-)
> > 
> > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > index 7c1f789..540dd67 100644
> > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
> >   	return mvdev->mlx_features;
> >   }
> > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
> > -{
> > -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > -		return -EOPNOTSUPP;
> > -
> > -	return 0;
> > -}
> > -
> >   static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> >   {
> >   	int err;
> > @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
> >   {
> >   	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> >   	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > -	int err;
> >   	print_features(mvdev, features, true);
> > -	err = verify_min_features(mvdev, features);
> > -	if (err)
> > -		return err;
> > -
> >   	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
> >   	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> >   	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
> > -	return err;
> > +	return 0;
> >   }
> >   static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)

Si-Wei Liu Feb. 22, 2021, 5:09 p.m. UTC | #6

On 2/21/2021 8:14 PM, Jason Wang wrote:
>
> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>> for legacy") made an exception for legacy guests to reset
>> features to 0, when config space is accessed before features
>> are set. We should relieve the verify_min_features() check
>> and allow features reset to 0 for this case.
>>
>> It's worth noting that not just legacy guests could access
>> config space before features are set. For instance, when
>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>> will try to access and validate the MTU present in the config
>> space before virtio features are set.
>
>
> This looks like a spec violation:
>
> "
>
> The following driver-read-only field, mtu only exists if 
> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the 
> driver to use.
> "
>
> Do we really want to workaround this?

Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?

I think the point is, since there's legacy guest we'd have to support, 
this host side workaround is unavoidable. Although I agree the violating 
driver should be fixed (yes, it's in today's upstream kernel which 
exists for a while now).

-Siwei

>
> Thanks
>
>
>> Rejecting reset to 0
>> prematurely causes correct MTU and link status unable to load
>> for the very first config space access, rendering issues like
>> guest showing inaccurate MTU value, or failure to reject
>> out-of-range MTU.
>>
>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 
>> devices")
>> Signed-off-by: Si-Wei Liu <[e-mail]>
>> ---
>>   drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>   1 file changed, 1 insertion(+), 14 deletions(-)
>>
>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c 
>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> index 7c1f789..540dd67 100644
>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct 
>> vdpa_device *vdev)
>>       return mvdev->mlx_features;
>>   }
>>   -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 
>> features)
>> -{
>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>> -        return -EOPNOTSUPP;
>> -
>> -    return 0;
>> -}
>> -
>>   static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>   {
>>       int err;
>> @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct 
>> vdpa_device *vdev, u64 features)
>>   {
>>       struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>       struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>> -    int err;
>>         print_features(mvdev, features, true);
>>   -    err = verify_min_features(mvdev, features);
>> -    if (err)
>> -        return err;
>> -
>>       ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
>>       ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>       ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, 
>> VIRTIO_NET_S_LINK_UP);
>> -    return err;
>> +    return 0;
>>   }
>>     static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, 
>> struct vdpa_callback *cb)
>

Si-Wei Liu Feb. 23, 2021, 1:12 a.m. UTC | #7

On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>> for legacy") made an exception for legacy guests to reset
>>> features to 0, when config space is accessed before features
>>> are set. We should relieve the verify_min_features() check
>>> and allow features reset to 0 for this case.
>>>
>>> It's worth noting that not just legacy guests could access
>>> config space before features are set. For instance, when
>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>> will try to access and validate the MTU present in the config
>>> space before virtio features are set.
>>
>> This looks like a spec violation:
>>
>> "
>>
>> The following driver-read-only field, mtu only exists if VIRTIO_NET_F_MTU is
>> set.
>> This field specifies the maximum MTU for the driver to use.
>> "
>>
>> Do we really want to workaround this?
>>
>> Thanks
> And also:
>
> The driver MUST follow this sequence to initialize a device:
> 1. Reset the device.
> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> fields to check that it can support the device before accepting it.
> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> support our subset of features and the device is unusable.
> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>
>
> so accessing config space before FEATURES_OK is a spec violation, right?
It is, but it's not relevant to what this commit tries to address. I 
thought the legacy guest still needs to be supported.

Having said, a separate patch has to be posted to fix the guest driver 
issue where this discrepancy is introduced to virtnet_validate() (since 
commit fe36cbe067). But it's not technically related to this patch.

-Siwei

>
>
>>> Rejecting reset to 0
>>> prematurely causes correct MTU and link status unable to load
>>> for the very first config space access, rendering issues like
>>> guest showing inaccurate MTU value, or failure to reject
>>> out-of-range MTU.
>>>
>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>> ---
>>>    drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>    1 file changed, 1 insertion(+), 14 deletions(-)
>>>
>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> index 7c1f789..540dd67 100644
>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>    	return mvdev->mlx_features;
>>>    }
>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
>>> -{
>>> -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>> -		return -EOPNOTSUPP;
>>> -
>>> -	return 0;
>>> -}
>>> -
>>>    static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>    {
>>>    	int err;
>>> @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
>>>    {
>>>    	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>    	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>> -	int err;
>>>    	print_features(mvdev, features, true);
>>> -	err = verify_min_features(mvdev, features);
>>> -	if (err)
>>> -		return err;
>>> -
>>>    	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
>>>    	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>    	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
>>> -	return err;
>>> +	return 0;
>>>    }
>>>    static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)

Jason Wang Feb. 23, 2021, 2:03 a.m. UTC | #8

On 2021/2/23 1:09 上午, Si-Wei Liu wrote:
>
>
> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>
>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>> for legacy") made an exception for legacy guests to reset
>>> features to 0, when config space is accessed before features
>>> are set. We should relieve the verify_min_features() check
>>> and allow features reset to 0 for this case.
>>>
>>> It's worth noting that not just legacy guests could access
>>> config space before features are set. For instance, when
>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>> will try to access and validate the MTU present in the config
>>> space before virtio features are set.
>>
>>
>> This looks like a spec violation:
>>
>> "
>>
>> The following driver-read-only field, mtu only exists if 
>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the 
>> driver to use.
>> "
>>
>> Do we really want to workaround this?
>
> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?


Yes, but the problem is we can't detect whether or not it's an legacy 
guest (e.g feature is not set).


>
> I think the point is, since there's legacy guest we'd have to support, 
> this host side workaround is unavoidable.


Since from vhost-vDPA point of view the driver is Qemu, it means we need 
make qemu vhost-vDPA driver spec complaint.

E.g how about:

1) revert 452639a64ad8 and fix Qemu? In Qemu, during vhost-vDPA 
initialization, do a minial config sync by neogitating minimal features 
(e.g just VIRTIO_F_ACCESS_PLATFORM). When FEATURE_OK is not set from 
guest, we can only allow it to access the config space that is emulated 
by Qemu?

Then


> Although I agree the violating driver should be fixed (yes, it's in 
> today's upstream kernel which exists for a while now).


2) Fix the virtio driver bug.

Or a quick workaround is to set (VIRTIO_F_ACCESS_PLATFORM instead of 0) 
in this case.

Thanks


>
> -Siwei
>
>>
>> Thanks
>>
>>
>>> Rejecting reset to 0
>>> prematurely causes correct MTU and link status unable to load
>>> for the very first config space access, rendering issues like
>>> guest showing inaccurate MTU value, or failure to reject
>>> out-of-range MTU.
>>>
>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 
>>> devices")
>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>> ---
>>>   drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>   1 file changed, 1 insertion(+), 14 deletions(-)
>>>
>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c 
>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> index 7c1f789..540dd67 100644
>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct 
>>> vdpa_device *vdev)
>>>       return mvdev->mlx_features;
>>>   }
>>>   -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 
>>> features)
>>> -{
>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>> -        return -EOPNOTSUPP;
>>> -
>>> -    return 0;
>>> -}
>>> -
>>>   static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>   {
>>>       int err;
>>> @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct 
>>> vdpa_device *vdev, u64 features)
>>>   {
>>>       struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>       struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>> -    int err;
>>>         print_features(mvdev, features, true);
>>>   -    err = verify_min_features(mvdev, features);
>>> -    if (err)
>>> -        return err;
>>> -
>>>       ndev->mvdev.actual_features = features & 
>>> ndev->mvdev.mlx_features;
>>>       ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>       ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, 
>>> VIRTIO_NET_S_LINK_UP);
>>> -    return err;
>>> +    return 0;
>>>   }
>>>     static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, 
>>> struct vdpa_callback *cb)
>>
>

Jason Wang Feb. 23, 2021, 2:03 a.m. UTC | #9

On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>
>
> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>> for legacy") made an exception for legacy guests to reset
>>>> features to 0, when config space is accessed before features
>>>> are set. We should relieve the verify_min_features() check
>>>> and allow features reset to 0 for this case.
>>>>
>>>> It's worth noting that not just legacy guests could access
>>>> config space before features are set. For instance, when
>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>> will try to access and validate the MTU present in the config
>>>> space before virtio features are set.
>>>
>>> This looks like a spec violation:
>>>
>>> "
>>>
>>> The following driver-read-only field, mtu only exists if 
>>> VIRTIO_NET_F_MTU is
>>> set.
>>> This field specifies the maximum MTU for the driver to use.
>>> "
>>>
>>> Do we really want to workaround this?
>>>
>>> Thanks
>> And also:
>>
>> The driver MUST follow this sequence to initialize a device:
>> 1. Reset the device.
>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>> 3. Set the DRIVER status bit: the guest OS knows how to drive the 
>> device.
>> 4. Read device feature bits, and write the subset of feature bits 
>> understood by the OS and driver to the
>> device. During this step the driver MAY read (but MUST NOT write) the 
>> device-specific configuration
>> fields to check that it can support the device before accepting it.
>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new 
>> feature bits after this step.
>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: 
>> otherwise, the device does not
>> support our subset of features and the device is unusable.
>> 7. Perform device-specific setup, including discovery of virtqueues 
>> for the device, optional per-bus setup,
>> reading and possibly writing the device’s virtio configuration space, 
>> and population of virtqueues.
>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>
>>
>> so accessing config space before FEATURES_OK is a spec violation, right?
> It is, but it's not relevant to what this commit tries to address. I 
> thought the legacy guest still needs to be supported.
>
> Having said, a separate patch has to be posted to fix the guest driver 
> issue where this discrepancy is introduced to virtnet_validate() 
> (since commit fe36cbe067). But it's not technically related to this 
> patch.
>
> -Siwei


I think it's a bug to read config space in validate, we should move it 
to virtnet_probe().

Thanks


>
>>
>>
>>>> Rejecting reset to 0
>>>> prematurely causes correct MTU and link status unable to load
>>>> for the very first config space access, rendering issues like
>>>> guest showing inaccurate MTU value, or failure to reject
>>>> out-of-range MTU.
>>>>
>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 
>>>> devices")
>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>> ---
>>>>    drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>    1 file changed, 1 insertion(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c 
>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>> index 7c1f789..540dd67 100644
>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct 
>>>> vdpa_device *vdev)
>>>>        return mvdev->mlx_features;
>>>>    }
>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 
>>>> features)
>>>> -{
>>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>> -        return -EOPNOTSUPP;
>>>> -
>>>> -    return 0;
>>>> -}
>>>> -
>>>>    static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>    {
>>>>        int err;
>>>> @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct 
>>>> vdpa_device *vdev, u64 features)
>>>>    {
>>>>        struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>        struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>> -    int err;
>>>>        print_features(mvdev, features, true);
>>>> -    err = verify_min_features(mvdev, features);
>>>> -    if (err)
>>>> -        return err;
>>>> -
>>>>        ndev->mvdev.actual_features = features & 
>>>> ndev->mvdev.mlx_features;
>>>>        ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>        ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, 
>>>> VIRTIO_NET_S_LINK_UP);
>>>> -    return err;
>>>> +    return 0;
>>>>    }
>>>>    static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, 
>>>> struct vdpa_callback *cb)
>

Michael S. Tsirkin Feb. 23, 2021, 9:25 a.m. UTC | #10

On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
> 
> 
> On 2/21/2021 8:14 PM, Jason Wang wrote:
> > 
> > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > for legacy") made an exception for legacy guests to reset
> > > features to 0, when config space is accessed before features
> > > are set. We should relieve the verify_min_features() check
> > > and allow features reset to 0 for this case.
> > > 
> > > It's worth noting that not just legacy guests could access
> > > config space before features are set. For instance, when
> > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > will try to access and validate the MTU present in the config
> > > space before virtio features are set.
> > 
> > 
> > This looks like a spec violation:
> > 
> > "
> > 
> > The following driver-read-only field, mtu only exists if
> > VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
> > driver to use.
> > "
> > 
> > Do we really want to workaround this?
> 
> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
> 
> I think the point is, since there's legacy guest we'd have to support, this
> host side workaround is unavoidable. Although I agree the violating driver
> should be fixed (yes, it's in today's upstream kernel which exists for a
> while now).

Oh  you are right:


static int virtnet_validate(struct virtio_device *vdev)
{
        if (!vdev->config->get) {
                dev_err(&vdev->dev, "%s failure: config access disabled\n",
                        __func__);
                return -EINVAL;
        }

        if (!virtnet_validate_features(vdev))
                return -EINVAL;

        if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
                int mtu = virtio_cread16(vdev,
                                         offsetof(struct virtio_net_config,
                                                  mtu));
                if (mtu < MIN_MTU)
                        __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
        }

        return 0;
}

And the spec says:


The driver MUST follow this sequence to initialize a device:
1. Reset the device.
2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
3. Set the DRIVER status bit: the guest OS knows how to drive the device.
4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
fields to check that it can support the device before accepting it.
5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
support our subset of features and the device is unusable.
7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
8. Set the DRIVER_OK status bit. At this point the device is “live”.


Item 4 on the list explicitly allows reading config space before
FEATURES_OK.

I conclude that VIRTIO_NET_F_MTU is set means "set in device features".

Generally it is worth going over feature dependent config fields
and checking whether they should be present when device feature is set
or when feature bit has been negotiated, and making this clear.

Michael S. Tsirkin Feb. 23, 2021, 9:26 a.m. UTC | #11

On Mon, Feb 22, 2021 at 08:05:26AM +0200, Eli Cohen wrote:
> On Sun, Feb 21, 2021 at 04:52:05PM -0500, Michael S. Tsirkin wrote:
> > On Sun, Feb 21, 2021 at 04:44:37PM +0200, Eli Cohen wrote:
> > > On Fri, Feb 19, 2021 at 06:54:58AM -0500, Si-Wei Liu wrote:
> > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > for legacy") made an exception for legacy guests to reset
> > > > features to 0, when config space is accessed before features
> > > > are set. We should relieve the verify_min_features() check
> > > > and allow features reset to 0 for this case.
> > > > 
> > > > It's worth noting that not just legacy guests could access
> > > > config space before features are set. For instance, when
> > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > will try to access and validate the MTU present in the config
> > > > space before virtio features are set. Rejecting reset to 0
> > > > prematurely causes correct MTU and link status unable to load
> > > > for the very first config space access, rendering issues like
> > > > guest showing inaccurate MTU value, or failure to reject
> > > > out-of-range MTU.
> > > > 
> > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > ---
> > > >  drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > >  1 file changed, 1 insertion(+), 14 deletions(-)
> > > > 
> > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > index 7c1f789..540dd67 100644
> > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > >  	return mvdev->mlx_features;
> > > >  }
> > > >  
> > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
> > > > -{
> > > > -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > -		return -EOPNOTSUPP;
> > > > -
> > > > -	return 0;
> > > > -}
> > > > -
> > > 
> > > But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
> > > support such cases.
> > 
> > Did you mean "catch such cases" rather than "support"?
> > 
> 
> Actually I meant this driver/device does not support such cases.

Well the removed code merely failed without VIRTIO_F_ACCESS_PLATFORM
it didn't actually try to support anything ...

> > 
> > > Maybe we should call verify_min_features() from mlx5_vdpa_set_status()
> > > just before attempting to call setup_driver().
> > > 
> > > >  static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > >  {
> > > >  	int err;
> > > > @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
> > > >  {
> > > >  	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > >  	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > -	int err;
> > > >  
> > > >  	print_features(mvdev, features, true);
> > > >  
> > > > -	err = verify_min_features(mvdev, features);
> > > > -	if (err)
> > > > -		return err;
> > > > -
> > > >  	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
> > > >  	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > >  	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
> > > > -	return err;
> > > > +	return 0;
> > > >  }
> > > >  
> > > >  static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)
> > > > -- 
> > > > 1.8.3.1
> > > > 
> >

Jason Wang Feb. 23, 2021, 9:46 a.m. UTC | #12

On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
>>
>> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>> for legacy") made an exception for legacy guests to reset
>>>> features to 0, when config space is accessed before features
>>>> are set. We should relieve the verify_min_features() check
>>>> and allow features reset to 0 for this case.
>>>>
>>>> It's worth noting that not just legacy guests could access
>>>> config space before features are set. For instance, when
>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>> will try to access and validate the MTU present in the config
>>>> space before virtio features are set.
>>>
>>> This looks like a spec violation:
>>>
>>> "
>>>
>>> The following driver-read-only field, mtu only exists if
>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
>>> driver to use.
>>> "
>>>
>>> Do we really want to workaround this?
>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
>>
>> I think the point is, since there's legacy guest we'd have to support, this
>> host side workaround is unavoidable. Although I agree the violating driver
>> should be fixed (yes, it's in today's upstream kernel which exists for a
>> while now).
> Oh  you are right:
>
>
> static int virtnet_validate(struct virtio_device *vdev)
> {
>          if (!vdev->config->get) {
>                  dev_err(&vdev->dev, "%s failure: config access disabled\n",
>                          __func__);
>                  return -EINVAL;
>          }
>
>          if (!virtnet_validate_features(vdev))
>                  return -EINVAL;
>
>          if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>                  int mtu = virtio_cread16(vdev,
>                                           offsetof(struct virtio_net_config,
>                                                    mtu));
>                  if (mtu < MIN_MTU)
>                          __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);


I wonder why not simply fail here?


>          }
>
>          return 0;
> }
>
> And the spec says:
>
>
> The driver MUST follow this sequence to initialize a device:
> 1. Reset the device.
> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> fields to check that it can support the device before accepting it.
> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> support our subset of features and the device is unusable.
> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>
>
> Item 4 on the list explicitly allows reading config space before
> FEATURES_OK.
>
> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".


So this probably need some clarification. "is set" is used many times in 
the spec that has different implications.

Thanks


>
> Generally it is worth going over feature dependent config fields
> and checking whether they should be present when device feature is set
> or when feature bit has been negotiated, and making this clear.
>

Jason Wang Feb. 23, 2021, 9:48 a.m. UTC | #13

On 2021/2/23 下午5:26, Michael S. Tsirkin wrote:
> On Mon, Feb 22, 2021 at 08:05:26AM +0200, Eli Cohen wrote:
>> On Sun, Feb 21, 2021 at 04:52:05PM -0500, Michael S. Tsirkin wrote:
>>> On Sun, Feb 21, 2021 at 04:44:37PM +0200, Eli Cohen wrote:
>>>> On Fri, Feb 19, 2021 at 06:54:58AM -0500, Si-Wei Liu wrote:
>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>> for legacy") made an exception for legacy guests to reset
>>>>> features to 0, when config space is accessed before features
>>>>> are set. We should relieve the verify_min_features() check
>>>>> and allow features reset to 0 for this case.
>>>>>
>>>>> It's worth noting that not just legacy guests could access
>>>>> config space before features are set. For instance, when
>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>> will try to access and validate the MTU present in the config
>>>>> space before virtio features are set. Rejecting reset to 0
>>>>> prematurely causes correct MTU and link status unable to load
>>>>> for the very first config space access, rendering issues like
>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>> out-of-range MTU.
>>>>>
>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
>>>>> Signed-off-by: Si-Wei Liu<[e-mail]>
>>>>> ---
>>>>>   drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>   1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>> index 7c1f789..540dd67 100644
>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>   	return mvdev->mlx_features;
>>>>>   }
>>>>>   
>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
>>>>> -{
>>>>> -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>> -		return -EOPNOTSUPP;
>>>>> -
>>>>> -	return 0;
>>>>> -}
>>>>> -
>>>> But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
>>>> support such cases.
>>> Did you mean "catch such cases" rather than "support"?
>>>
>> Actually I meant this driver/device does not support such cases.
> Well the removed code merely failed without VIRTIO_F_ACCESS_PLATFORM
> it didn't actually try to support anything ...


I think it's used to catch the driver that doesn't support ACCESS_PLATFORM?

Thanks


>

Michael S. Tsirkin Feb. 23, 2021, 9:55 a.m. UTC | #14

On Tue, Feb 23, 2021 at 05:48:10PM +0800, Jason Wang wrote:
> 
> On 2021/2/23 下午5:26, Michael S. Tsirkin wrote:
> > On Mon, Feb 22, 2021 at 08:05:26AM +0200, Eli Cohen wrote:
> > > On Sun, Feb 21, 2021 at 04:52:05PM -0500, Michael S. Tsirkin wrote:
> > > > On Sun, Feb 21, 2021 at 04:44:37PM +0200, Eli Cohen wrote:
> > > > > On Fri, Feb 19, 2021 at 06:54:58AM -0500, Si-Wei Liu wrote:
> > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > features to 0, when config space is accessed before features
> > > > > > are set. We should relieve the verify_min_features() check
> > > > > > and allow features reset to 0 for this case.
> > > > > > 
> > > > > > It's worth noting that not just legacy guests could access
> > > > > > config space before features are set. For instance, when
> > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > will try to access and validate the MTU present in the config
> > > > > > space before virtio features are set. Rejecting reset to 0
> > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > for the very first config space access, rendering issues like
> > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > out-of-range MTU.
> > > > > > 
> > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > > > > > Signed-off-by: Si-Wei Liu<[e-mail]>
> > > > > > ---
> > > > > >   drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > >   1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > index 7c1f789..540dd67 100644
> > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > >   	return mvdev->mlx_features;
> > > > > >   }
> > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
> > > > > > -{
> > > > > > -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > -		return -EOPNOTSUPP;
> > > > > > -
> > > > > > -	return 0;
> > > > > > -}
> > > > > > -
> > > > > But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
> > > > > support such cases.
> > > > Did you mean "catch such cases" rather than "support"?
> > > > 
> > > Actually I meant this driver/device does not support such cases.
> > Well the removed code merely failed without VIRTIO_F_ACCESS_PLATFORM
> > it didn't actually try to support anything ...
> 
> 
> I think it's used to catch the driver that doesn't support ACCESS_PLATFORM?
> 
> Thanks
> 

That is why I asked whether Eli meant catch.

Michael S. Tsirkin Feb. 23, 2021, 10:01 a.m. UTC | #15

On Tue, Feb 23, 2021 at 05:46:20PM +0800, Jason Wang wrote:
> 
> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
> > On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
> > > 
> > > On 2/21/2021 8:14 PM, Jason Wang wrote:
> > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > for legacy") made an exception for legacy guests to reset
> > > > > features to 0, when config space is accessed before features
> > > > > are set. We should relieve the verify_min_features() check
> > > > > and allow features reset to 0 for this case.
> > > > > 
> > > > > It's worth noting that not just legacy guests could access
> > > > > config space before features are set. For instance, when
> > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > will try to access and validate the MTU present in the config
> > > > > space before virtio features are set.
> > > > 
> > > > This looks like a spec violation:
> > > > 
> > > > "
> > > > 
> > > > The following driver-read-only field, mtu only exists if
> > > > VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
> > > > driver to use.
> > > > "
> > > > 
> > > > Do we really want to workaround this?
> > > Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
> > > 
> > > I think the point is, since there's legacy guest we'd have to support, this
> > > host side workaround is unavoidable. Although I agree the violating driver
> > > should be fixed (yes, it's in today's upstream kernel which exists for a
> > > while now).
> > Oh  you are right:
> > 
> > 
> > static int virtnet_validate(struct virtio_device *vdev)
> > {
> >          if (!vdev->config->get) {
> >                  dev_err(&vdev->dev, "%s failure: config access disabled\n",
> >                          __func__);
> >                  return -EINVAL;
> >          }
> > 
> >          if (!virtnet_validate_features(vdev))
> >                  return -EINVAL;
> > 
> >          if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> >                  int mtu = virtio_cread16(vdev,
> >                                           offsetof(struct virtio_net_config,
> >                                                    mtu));
> >                  if (mtu < MIN_MTU)
> >                          __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
> 
> 
> I wonder why not simply fail here?

Back in 2016 it went like this:

	On Thu, Jun 02, 2016 at 05:10:59PM -0400, Aaron Conole wrote:
	> +     if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
	> +             dev->mtu = virtio_cread16(vdev,
	> +                                       offsetof(struct virtio_net_config,
	> +                                                mtu));
	> +     }
	> +
	>       if (vi->any_header_sg)
	>               dev->needed_headroom = vi->hdr_len;
	> 

	One comment though: I think we should validate the mtu.
	If it's invalid, clear VIRTIO_NET_F_MTU and ignore.


Too late at this point :)

I guess it's a way to tell device "I can not live with this MTU",
device can fail FEATURES_OK if it wants to. MIN_MTU
is an internal linux thing and at the time I felt it's better to
try to make progress.


> 
> >          }
> > 
> >          return 0;
> > }
> > 
> > And the spec says:
> > 
> > 
> > The driver MUST follow this sequence to initialize a device:
> > 1. Reset the device.
> > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> > 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> > device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> > fields to check that it can support the device before accepting it.
> > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> > 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> > support our subset of features and the device is unusable.
> > 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> > reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > 
> > 
> > Item 4 on the list explicitly allows reading config space before
> > FEATURES_OK.
> > 
> > I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
> 
> 
> So this probably need some clarification. "is set" is used many times in the
> spec that has different implications.
> 
> Thanks
> 
> 
> > 
> > Generally it is worth going over feature dependent config fields
> > and checking whether they should be present when device feature is set
> > or when feature bit has been negotiated, and making this clear.
> >

Cornelia Huck Feb. 23, 2021, 10:04 a.m. UTC | #16

On Tue, 23 Feb 2021 17:46:20 +0800
Jason Wang <[e-mail]> wrote:

> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
> > On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:  
> >>
> >> On 2/21/2021 8:14 PM, Jason Wang wrote:  
> >>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:  
> >>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> >>>> for legacy") made an exception for legacy guests to reset
> >>>> features to 0, when config space is accessed before features
> >>>> are set. We should relieve the verify_min_features() check
> >>>> and allow features reset to 0 for this case.
> >>>>
> >>>> It's worth noting that not just legacy guests could access
> >>>> config space before features are set. For instance, when
> >>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
> >>>> will try to access and validate the MTU present in the config
> >>>> space before virtio features are set.  
> >>>
> >>> This looks like a spec violation:
> >>>
> >>> "
> >>>
> >>> The following driver-read-only field, mtu only exists if
> >>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
> >>> driver to use.
> >>> "
> >>>
> >>> Do we really want to workaround this?  
> >> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
> >>
> >> I think the point is, since there's legacy guest we'd have to support, this
> >> host side workaround is unavoidable. Although I agree the violating driver
> >> should be fixed (yes, it's in today's upstream kernel which exists for a
> >> while now).  
> > Oh  you are right:
> >
> >
> > static int virtnet_validate(struct virtio_device *vdev)
> > {
> >          if (!vdev->config->get) {
> >                  dev_err(&vdev->dev, "%s failure: config access disabled\n",
> >                          __func__);
> >                  return -EINVAL;
> >          }
> >
> >          if (!virtnet_validate_features(vdev))
> >                  return -EINVAL;
> >
> >          if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> >                  int mtu = virtio_cread16(vdev,
> >                                           offsetof(struct virtio_net_config,
> >                                                    mtu));
> >                  if (mtu < MIN_MTU)
> >                          __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);  
> 
> 
> I wonder why not simply fail here?

I think both failing or not accepting the feature can be argued to make
sense: "the device presented us with a mtu size that does not make
sense" would point to failing, "we cannot work with the mtu size that
the device presented us" would point to not negotiating the feature.

> 
> 
> >          }
> >
> >          return 0;
> > }
> >
> > And the spec says:
> >
> >
> > The driver MUST follow this sequence to initialize a device:
> > 1. Reset the device.
> > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> > 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> > device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> > fields to check that it can support the device before accepting it.
> > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> > 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> > support our subset of features and the device is unusable.
> > 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> > reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> >
> >
> > Item 4 on the list explicitly allows reading config space before
> > FEATURES_OK.
> >
> > I conclude that VIRTIO_NET_F_MTU is set means "set in device features".  
> 
> 
> So this probably need some clarification. "is set" is used many times in 
> the spec that has different implications.

Before FEATURES_OK is set by the driver, I guess it means "the device
has offered the feature"; during normal usage, it means "the feature
has been negotiated". (This is a bit fuzzy for legacy mode.)

Should we add a wording clarification to the spec?

Jason Wang Feb. 23, 2021, 10:17 a.m. UTC | #17

On 2021/2/23 6:01 下午, Michael S. Tsirkin wrote:
> On Tue, Feb 23, 2021 at 05:46:20PM +0800, Jason Wang wrote:
>> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
>>>> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>> features to 0, when config space is accessed before features
>>>>>> are set. We should relieve the verify_min_features() check
>>>>>> and allow features reset to 0 for this case.
>>>>>>
>>>>>> It's worth noting that not just legacy guests could access
>>>>>> config space before features are set. For instance, when
>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>> will try to access and validate the MTU present in the config
>>>>>> space before virtio features are set.
>>>>> This looks like a spec violation:
>>>>>
>>>>> "
>>>>>
>>>>> The following driver-read-only field, mtu only exists if
>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
>>>>> driver to use.
>>>>> "
>>>>>
>>>>> Do we really want to workaround this?
>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
>>>>
>>>> I think the point is, since there's legacy guest we'd have to support, this
>>>> host side workaround is unavoidable. Although I agree the violating driver
>>>> should be fixed (yes, it's in today's upstream kernel which exists for a
>>>> while now).
>>> Oh  you are right:
>>>
>>>
>>> static int virtnet_validate(struct virtio_device *vdev)
>>> {
>>>           if (!vdev->config->get) {
>>>                   dev_err(&vdev->dev, "%s failure: config access disabled\n",
>>>                           __func__);
>>>                   return -EINVAL;
>>>           }
>>>
>>>           if (!virtnet_validate_features(vdev))
>>>                   return -EINVAL;
>>>
>>>           if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>>                   int mtu = virtio_cread16(vdev,
>>>                                            offsetof(struct virtio_net_config,
>>>                                                     mtu));
>>>                   if (mtu < MIN_MTU)
>>>                           __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>
>> I wonder why not simply fail here?
> Back in 2016 it went like this:
>
> 	On Thu, Jun 02, 2016 at 05:10:59PM -0400, Aaron Conole wrote:
> 	> +     if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> 	> +             dev->mtu = virtio_cread16(vdev,
> 	> +                                       offsetof(struct virtio_net_config,
> 	> +                                                mtu));
> 	> +     }
> 	> +
> 	>       if (vi->any_header_sg)
> 	>               dev->needed_headroom = vi->hdr_len;
> 	>
>
> 	One comment though: I think we should validate the mtu.
> 	If it's invalid, clear VIRTIO_NET_F_MTU and ignore.
>
>
> Too late at this point :)
>
> I guess it's a way to tell device "I can not live with this MTU",
> device can fail FEATURES_OK if it wants to. MIN_MTU
> is an internal linux thing and at the time I felt it's better to
> try to make progress.


What if e.g the device advertise a large MTU. E.g 64K here? In that 
case, the driver can not live either. Clearing MTU won't help here.

Thanks


>
>
>>>           }
>>>
>>>           return 0;
>>> }
>>>
>>> And the spec says:
>>>
>>>
>>> The driver MUST follow this sequence to initialize a device:
>>> 1. Reset the device.
>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
>>> fields to check that it can support the device before accepting it.
>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
>>> support our subset of features and the device is unusable.
>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
>>> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>
>>>
>>> Item 4 on the list explicitly allows reading config space before
>>> FEATURES_OK.
>>>
>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
>>
>> So this probably need some clarification. "is set" is used many times in the
>> spec that has different implications.
>>
>> Thanks
>>
>>
>>> Generally it is worth going over feature dependent config fields
>>> and checking whether they should be present when device feature is set
>>> or when feature bit has been negotiated, and making this clear.
>>>

Jason Wang Feb. 23, 2021, 10:31 a.m. UTC | #18

On 2021/2/23 6:04 下午, Cornelia Huck wrote:
> On Tue, 23 Feb 2021 17:46:20 +0800
> Jason Wang <[e-mail]> wrote:
>
>> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
>>>> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>> features to 0, when config space is accessed before features
>>>>>> are set. We should relieve the verify_min_features() check
>>>>>> and allow features reset to 0 for this case.
>>>>>>
>>>>>> It's worth noting that not just legacy guests could access
>>>>>> config space before features are set. For instance, when
>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>> will try to access and validate the MTU present in the config
>>>>>> space before virtio features are set.
>>>>> This looks like a spec violation:
>>>>>
>>>>> "
>>>>>
>>>>> The following driver-read-only field, mtu only exists if
>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
>>>>> driver to use.
>>>>> "
>>>>>
>>>>> Do we really want to workaround this?
>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
>>>>
>>>> I think the point is, since there's legacy guest we'd have to support, this
>>>> host side workaround is unavoidable. Although I agree the violating driver
>>>> should be fixed (yes, it's in today's upstream kernel which exists for a
>>>> while now).
>>> Oh  you are right:
>>>
>>>
>>> static int virtnet_validate(struct virtio_device *vdev)
>>> {
>>>           if (!vdev->config->get) {
>>>                   dev_err(&vdev->dev, "%s failure: config access disabled\n",
>>>                           __func__);
>>>                   return -EINVAL;
>>>           }
>>>
>>>           if (!virtnet_validate_features(vdev))
>>>                   return -EINVAL;
>>>
>>>           if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>>                   int mtu = virtio_cread16(vdev,
>>>                                            offsetof(struct virtio_net_config,
>>>                                                     mtu));
>>>                   if (mtu < MIN_MTU)
>>>                           __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>
>> I wonder why not simply fail here?
> I think both failing or not accepting the feature can be argued to make
> sense: "the device presented us with a mtu size that does not make
> sense" would point to failing, "we cannot work with the mtu size that
> the device presented us" would point to not negotiating the feature.
>
>>
>>>           }
>>>
>>>           return 0;
>>> }
>>>
>>> And the spec says:
>>>
>>>
>>> The driver MUST follow this sequence to initialize a device:
>>> 1. Reset the device.
>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
>>> fields to check that it can support the device before accepting it.
>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
>>> support our subset of features and the device is unusable.
>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
>>> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>
>>>
>>> Item 4 on the list explicitly allows reading config space before
>>> FEATURES_OK.
>>>
>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
>>
>> So this probably need some clarification. "is set" is used many times in
>> the spec that has different implications.
> Before FEATURES_OK is set by the driver, I guess it means "the device
> has offered the feature";


For me this part is ok since it clarify that it's the driver that set 
the bit.



> during normal usage, it means "the feature
> has been negotiated".

/?

It looks to me the feature negotiation is done only after device set 
FEATURES_OK, or FEATURES_OK could be read from device status?


>   (This is a bit fuzzy for legacy mode.)


The problem is the MTU description for example:

"The following driver-read-only field, mtu only exists if 
VIRTIO_NET_F_MTU is set."

It looks to me need to use "if VIRTIO_NET_F_MTU is set by device". 
Otherwise readers (at least for me), may think the MTU is only valid if 
driver set the bit.


>
> Should we add a wording clarification to the spec?


I think so.

Thanks

>

Cornelia Huck Feb. 23, 2021, 10:58 a.m. UTC | #19

On Tue, 23 Feb 2021 18:31:07 +0800
Jason Wang <[e-mail]> wrote:

> On 2021/2/23 6:04 下午, Cornelia Huck wrote:
> > On Tue, 23 Feb 2021 17:46:20 +0800
> > Jason Wang <[e-mail]> wrote:
> >  
> >> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:  
> >>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:  
> >>>> On 2/21/2021 8:14 PM, Jason Wang wrote:  
> >>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:  
> >>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> >>>>>> for legacy") made an exception for legacy guests to reset
> >>>>>> features to 0, when config space is accessed before features
> >>>>>> are set. We should relieve the verify_min_features() check
> >>>>>> and allow features reset to 0 for this case.
> >>>>>>
> >>>>>> It's worth noting that not just legacy guests could access
> >>>>>> config space before features are set. For instance, when
> >>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
> >>>>>> will try to access and validate the MTU present in the config
> >>>>>> space before virtio features are set.  
> >>>>> This looks like a spec violation:
> >>>>>
> >>>>> "
> >>>>>
> >>>>> The following driver-read-only field, mtu only exists if
> >>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
> >>>>> driver to use.
> >>>>> "
> >>>>>
> >>>>> Do we really want to workaround this?  
> >>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
> >>>>
> >>>> I think the point is, since there's legacy guest we'd have to support, this
> >>>> host side workaround is unavoidable. Although I agree the violating driver
> >>>> should be fixed (yes, it's in today's upstream kernel which exists for a
> >>>> while now).  
> >>> Oh  you are right:
> >>>
> >>>
> >>> static int virtnet_validate(struct virtio_device *vdev)
> >>> {
> >>>           if (!vdev->config->get) {
> >>>                   dev_err(&vdev->dev, "%s failure: config access disabled\n",
> >>>                           __func__);
> >>>                   return -EINVAL;
> >>>           }
> >>>
> >>>           if (!virtnet_validate_features(vdev))
> >>>                   return -EINVAL;
> >>>
> >>>           if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> >>>                   int mtu = virtio_cread16(vdev,
> >>>                                            offsetof(struct virtio_net_config,
> >>>                                                     mtu));
> >>>                   if (mtu < MIN_MTU)
> >>>                           __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);  
> >>
> >> I wonder why not simply fail here?  
> > I think both failing or not accepting the feature can be argued to make
> > sense: "the device presented us with a mtu size that does not make
> > sense" would point to failing, "we cannot work with the mtu size that
> > the device presented us" would point to not negotiating the feature.
> >  
> >>  
> >>>           }
> >>>
> >>>           return 0;
> >>> }
> >>>
> >>> And the spec says:
> >>>
> >>>
> >>> The driver MUST follow this sequence to initialize a device:
> >>> 1. Reset the device.
> >>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> >>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> >>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> >>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> >>> fields to check that it can support the device before accepting it.
> >>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> >>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> >>> support our subset of features and the device is unusable.
> >>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> >>> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> >>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> >>>
> >>>
> >>> Item 4 on the list explicitly allows reading config space before
> >>> FEATURES_OK.
> >>>
> >>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".  
> >>
> >> So this probably need some clarification. "is set" is used many times in
> >> the spec that has different implications.  
> > Before FEATURES_OK is set by the driver, I guess it means "the device
> > has offered the feature";  
> 
> 
> For me this part is ok since it clarify that it's the driver that set 
> the bit.
> 
> 
> 
> > during normal usage, it means "the feature
> > has been negotiated".  
> 
> /?
> 
> It looks to me the feature negotiation is done only after device set 
> FEATURES_OK, or FEATURES_OK could be read from device status?

I'd consider feature negotiation done when the driver reads FEATURES_OK
back from the status.

> 
> 
> >   (This is a bit fuzzy for legacy mode.)

...because legacy does not have FEATURES_OK.
  
> 
> 
> The problem is the MTU description for example:
> 
> "The following driver-read-only field, mtu only exists if 
> VIRTIO_NET_F_MTU is set."
> 
> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".

"offered by the device"? I don't think it should 'disappear' from the
config space if the driver won't use it. (Same for other config space
fields that are tied to feature bits.)
 
> Otherwise readers (at least for me), may think the MTU is only valid
> if driver set the bit.

I think it would still be 'valid' in the sense that it exists and has
some value in there filled in by the device, but a driver reading it
without negotiating the feature would be buggy. (Like in the kernel
code above; the kernel not liking the value does not make the field
invalid.)

Maybe a statement covering everything would be:

"The following driver-read-only field mtu only exists if the device
offers VIRTIO_NET_F_MTU and may be read by the driver during feature
negotiation and after VIRTIO_NET_F_MTU has been successfully
negotiated."

> 
> 
> >
> > Should we add a wording clarification to the spec?  
> 
> 
> I think so.

Some clarification would be needed for each field that depends on a
feature; that would be quite verbose. Maybe we can get away with a
clarifying statement?

"Some config space fields may depend on a certain feature. In that
case, the field exits if the device has offered the corresponding
feature, and may be read by the driver during feature negotiation, and
accessed by the driver after the feature has been successfully
negotiated. A shorthand for this is a statement that a field only
exists if a certain feature bit is set."

Michael S. Tsirkin Feb. 23, 2021, 12:26 p.m. UTC | #20

On Fri, Feb 19, 2021 at 06:54:58AM -0500, Si-Wei Liu wrote:
> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> for legacy") made an exception for legacy guests to reset
> features to 0, when config space is accessed before features
> are set. We should relieve the verify_min_features() check
> and allow features reset to 0 for this case.
> 
> It's worth noting that not just legacy guests could access
> config space before features are set. For instance, when
> feature VIRTIO_NET_F_MTU is advertised some modern driver
> will try to access and validate the MTU present in the config
> space before virtio features are set. Rejecting reset to 0
> prematurely causes correct MTU and link status unable to load
> for the very first config space access, rendering issues like
> guest showing inaccurate MTU value, or failure to reject
> out-of-range MTU.
> 
> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")

isn't this more

    vdpa: make sure set_features is invoked for legacy


> Signed-off-by: Si-Wei Liu <[e-mail]>

I think we at least need to correct the comment in
include/linux/vdpa.h then

Instead of "we assume a legacy guest" we'd say something like
"call set features in case it's a legacy guest".

Generally it's unfortunate. Need to think about what to do here.
Any idea how else we can cleanly detect a legacy guest?

> ---
>  drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>  1 file changed, 1 insertion(+), 14 deletions(-)
> 
> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> index 7c1f789..540dd67 100644
> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
>  	return mvdev->mlx_features;
>  }
>  
> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
> -{
> -	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> -		return -EOPNOTSUPP;
> -
> -	return 0;
> -}
> -
>  static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>  {
>  	int err;

Let's just set VIRTIO_F_ACCESS_PLATFORM in core?
Then we don't need to hack mlx5 ...


> @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
>  {
>  	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>  	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> -	int err;
>  
>  	print_features(mvdev, features, true);
>  
> -	err = verify_min_features(mvdev, features);
> -	if (err)
> -		return err;
> -
>  	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
>  	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>  	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
> -	return err;
> +	return 0;
>  }
>  
>  static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)
> -- 
> 1.8.3.1

Michael S. Tsirkin Feb. 23, 2021, 1:26 p.m. UTC | #21

On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> 
> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > 
> > 
> > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > for legacy") made an exception for legacy guests to reset
> > > > > features to 0, when config space is accessed before features
> > > > > are set. We should relieve the verify_min_features() check
> > > > > and allow features reset to 0 for this case.
> > > > > 
> > > > > It's worth noting that not just legacy guests could access
> > > > > config space before features are set. For instance, when
> > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > will try to access and validate the MTU present in the config
> > > > > space before virtio features are set.
> > > > 
> > > > This looks like a spec violation:
> > > > 
> > > > "
> > > > 
> > > > The following driver-read-only field, mtu only exists if
> > > > VIRTIO_NET_F_MTU is
> > > > set.
> > > > This field specifies the maximum MTU for the driver to use.
> > > > "
> > > > 
> > > > Do we really want to workaround this?
> > > > 
> > > > Thanks
> > > And also:
> > > 
> > > The driver MUST follow this sequence to initialize a device:
> > > 1. Reset the device.
> > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > device.
> > > 4. Read device feature bits, and write the subset of feature bits
> > > understood by the OS and driver to the
> > > device. During this step the driver MAY read (but MUST NOT write)
> > > the device-specific configuration
> > > fields to check that it can support the device before accepting it.
> > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > feature bits after this step.
> > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > otherwise, the device does not
> > > support our subset of features and the device is unusable.
> > > 7. Perform device-specific setup, including discovery of virtqueues
> > > for the device, optional per-bus setup,
> > > reading and possibly writing the device’s virtio configuration
> > > space, and population of virtqueues.
> > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > 
> > > 
> > > so accessing config space before FEATURES_OK is a spec violation, right?
> > It is, but it's not relevant to what this commit tries to address. I
> > thought the legacy guest still needs to be supported.
> > 
> > Having said, a separate patch has to be posted to fix the guest driver
> > issue where this discrepancy is introduced to virtnet_validate() (since
> > commit fe36cbe067). But it's not technically related to this patch.
> > 
> > -Siwei
> 
> 
> I think it's a bug to read config space in validate, we should move it to
> virtnet_probe().
> 
> Thanks

I take it back, reading but not writing seems to be explicitly allowed by spec.
So our way to detect a legacy guest is bogus, need to think what is
the best way to handle this.

> 
> > 
> > > 
> > > 
> > > > > Rejecting reset to 0
> > > > > prematurely causes correct MTU and link status unable to load
> > > > > for the very first config space access, rendering issues like
> > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > out-of-range MTU.
> > > > > 
> > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > supported mlx5 devices")
> > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > ---
> > > > >    drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > >    1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > index 7c1f789..540dd67 100644
> > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > >        return mvdev->mlx_features;
> > > > >    }
> > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > u64 features)
> > > > > -{
> > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > -        return -EOPNOTSUPP;
> > > > > -
> > > > > -    return 0;
> > > > > -}
> > > > > -
> > > > >    static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > >    {
> > > > >        int err;
> > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > features)
> > > > >    {
> > > > >        struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > >        struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > -    int err;
> > > > >        print_features(mvdev, features, true);
> > > > > -    err = verify_min_features(mvdev, features);
> > > > > -    if (err)
> > > > > -        return err;
> > > > > -
> > > > >        ndev->mvdev.actual_features = features &
> > > > > ndev->mvdev.mlx_features;
> > > > >        ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > >        ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > VIRTIO_NET_S_LINK_UP);
> > > > > -    return err;
> > > > > +    return 0;
> > > > >    }
> > > > >    static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > *vdev, struct vdpa_callback *cb)
> >

Si-Wei Liu Feb. 23, 2021, 7:35 p.m. UTC | #22

On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
>> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>>>
>>> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>>>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>> features to 0, when config space is accessed before features
>>>>>> are set. We should relieve the verify_min_features() check
>>>>>> and allow features reset to 0 for this case.
>>>>>>
>>>>>> It's worth noting that not just legacy guests could access
>>>>>> config space before features are set. For instance, when
>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>> will try to access and validate the MTU present in the config
>>>>>> space before virtio features are set.
>>>>> This looks like a spec violation:
>>>>>
>>>>> "
>>>>>
>>>>> The following driver-read-only field, mtu only exists if
>>>>> VIRTIO_NET_F_MTU is
>>>>> set.
>>>>> This field specifies the maximum MTU for the driver to use.
>>>>> "
>>>>>
>>>>> Do we really want to workaround this?
>>>>>
>>>>> Thanks
>>>> And also:
>>>>
>>>> The driver MUST follow this sequence to initialize a device:
>>>> 1. Reset the device.
>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the
>>>> device.
>>>> 4. Read device feature bits, and write the subset of feature bits
>>>> understood by the OS and driver to the
>>>> device. During this step the driver MAY read (but MUST NOT write)
>>>> the device-specific configuration
>>>> fields to check that it can support the device before accepting it.
>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
>>>> feature bits after this step.
>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set:
>>>> otherwise, the device does not
>>>> support our subset of features and the device is unusable.
>>>> 7. Perform device-specific setup, including discovery of virtqueues
>>>> for the device, optional per-bus setup,
>>>> reading and possibly writing the device’s virtio configuration
>>>> space, and population of virtqueues.
>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>
>>>>
>>>> so accessing config space before FEATURES_OK is a spec violation, right?
>>> It is, but it's not relevant to what this commit tries to address. I
>>> thought the legacy guest still needs to be supported.
>>>
>>> Having said, a separate patch has to be posted to fix the guest driver
>>> issue where this discrepancy is introduced to virtnet_validate() (since
>>> commit fe36cbe067). But it's not technically related to this patch.
>>>
>>> -Siwei
>>
>> I think it's a bug to read config space in validate, we should move it to
>> virtnet_probe().
>>
>> Thanks
> I take it back, reading but not writing seems to be explicitly allowed by spec.
> So our way to detect a legacy guest is bogus, need to think what is
> the best way to handle this.
Then maybe revert commit fe36cbe067 and friends, and have QEMU detect 
legacy guest? Supposedly only config space write access needs to be 
guarded before setting FEATURES_OK.

-Siwie

>>>>
>>>>>> Rejecting reset to 0
>>>>>> prematurely causes correct MTU and link status unable to load
>>>>>> for the very first config space access, rendering issues like
>>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>>> out-of-range MTU.
>>>>>>
>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
>>>>>> supported mlx5 devices")
>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>> ---
>>>>>>     drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>>     1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>> index 7c1f789..540dd67 100644
>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>> @@ -1490,14 +1490,6 @@ static u64
>>>>>> mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>>         return mvdev->mlx_features;
>>>>>>     }
>>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
>>>>>> u64 features)
>>>>>> -{
>>>>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>>> -        return -EOPNOTSUPP;
>>>>>> -
>>>>>> -    return 0;
>>>>>> -}
>>>>>> -
>>>>>>     static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>>>     {
>>>>>>         int err;
>>>>>> @@ -1558,18 +1550,13 @@ static int
>>>>>> mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
>>>>>> features)
>>>>>>     {
>>>>>>         struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>>>         struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>>>> -    int err;
>>>>>>         print_features(mvdev, features, true);
>>>>>> -    err = verify_min_features(mvdev, features);
>>>>>> -    if (err)
>>>>>> -        return err;
>>>>>> -
>>>>>>         ndev->mvdev.actual_features = features &
>>>>>> ndev->mvdev.mlx_features;
>>>>>>         ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>>>         ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
>>>>>> VIRTIO_NET_S_LINK_UP);
>>>>>> -    return err;
>>>>>> +    return 0;
>>>>>>     }
>>>>>>     static void mlx5_vdpa_set_config_cb(struct vdpa_device
>>>>>> *vdev, struct vdpa_callback *cb)

Jason Wang Feb. 24, 2021, 3:20 a.m. UTC | #23

On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
>
>
> On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
>> On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
>>> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>>>>
>>>> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>>>>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>> features to 0, when config space is accessed before features
>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>> and allow features reset to 0 for this case.
>>>>>>>
>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>> config space before features are set. For instance, when
>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>> will try to access and validate the MTU present in the config
>>>>>>> space before virtio features are set.
>>>>>> This looks like a spec violation:
>>>>>>
>>>>>> "
>>>>>>
>>>>>> The following driver-read-only field, mtu only exists if
>>>>>> VIRTIO_NET_F_MTU is
>>>>>> set.
>>>>>> This field specifies the maximum MTU for the driver to use.
>>>>>> "
>>>>>>
>>>>>> Do we really want to workaround this?
>>>>>>
>>>>>> Thanks
>>>>> And also:
>>>>>
>>>>> The driver MUST follow this sequence to initialize a device:
>>>>> 1. Reset the device.
>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the 
>>>>> device.
>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the
>>>>> device.
>>>>> 4. Read device feature bits, and write the subset of feature bits
>>>>> understood by the OS and driver to the
>>>>> device. During this step the driver MAY read (but MUST NOT write)
>>>>> the device-specific configuration
>>>>> fields to check that it can support the device before accepting it.
>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
>>>>> feature bits after this step.
>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set:
>>>>> otherwise, the device does not
>>>>> support our subset of features and the device is unusable.
>>>>> 7. Perform device-specific setup, including discovery of virtqueues
>>>>> for the device, optional per-bus setup,
>>>>> reading and possibly writing the device’s virtio configuration
>>>>> space, and population of virtqueues.
>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>
>>>>>
>>>>> so accessing config space before FEATURES_OK is a spec violation, 
>>>>> right?
>>>> It is, but it's not relevant to what this commit tries to address. I
>>>> thought the legacy guest still needs to be supported.
>>>>
>>>> Having said, a separate patch has to be posted to fix the guest driver
>>>> issue where this discrepancy is introduced to virtnet_validate() 
>>>> (since
>>>> commit fe36cbe067). But it's not technically related to this patch.
>>>>
>>>> -Siwei
>>>
>>> I think it's a bug to read config space in validate, we should move 
>>> it to
>>> virtnet_probe().
>>>
>>> Thanks
>> I take it back, reading but not writing seems to be explicitly 
>> allowed by spec.
>> So our way to detect a legacy guest is bogus, need to think what is
>> the best way to handle this.
> Then maybe revert commit fe36cbe067 and friends, and have QEMU detect 
> legacy guest? Supposedly only config space write access needs to be 
> guarded before setting FEATURES_OK.


I agree. My understanding is that all vDPA must be modern device (since 
VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.

Thanks


>
> -Siwie
>
>>>>>
>>>>>>> Rejecting reset to 0
>>>>>>> prematurely causes correct MTU and link status unable to load
>>>>>>> for the very first config space access, rendering issues like
>>>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>>>> out-of-range MTU.
>>>>>>>
>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
>>>>>>> supported mlx5 devices")
>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>> ---
>>>>>>>     drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>>>     1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>> index 7c1f789..540dd67 100644
>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>> @@ -1490,14 +1490,6 @@ static u64
>>>>>>> mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>>>         return mvdev->mlx_features;
>>>>>>>     }
>>>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
>>>>>>> u64 features)
>>>>>>> -{
>>>>>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>>>> -        return -EOPNOTSUPP;
>>>>>>> -
>>>>>>> -    return 0;
>>>>>>> -}
>>>>>>> -
>>>>>>>     static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>>>>     {
>>>>>>>         int err;
>>>>>>> @@ -1558,18 +1550,13 @@ static int
>>>>>>> mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
>>>>>>> features)
>>>>>>>     {
>>>>>>>         struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>>>>         struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>>>>> -    int err;
>>>>>>>         print_features(mvdev, features, true);
>>>>>>> -    err = verify_min_features(mvdev, features);
>>>>>>> -    if (err)
>>>>>>> -        return err;
>>>>>>> -
>>>>>>>         ndev->mvdev.actual_features = features &
>>>>>>> ndev->mvdev.mlx_features;
>>>>>>>         ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>>>>         ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
>>>>>>> VIRTIO_NET_S_LINK_UP);
>>>>>>> -    return err;
>>>>>>> +    return 0;
>>>>>>>     }
>>>>>>>     static void mlx5_vdpa_set_config_cb(struct vdpa_device
>>>>>>> *vdev, struct vdpa_callback *cb)
>

Michael S. Tsirkin Feb. 24, 2021, 5:04 a.m. UTC | #24

On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
> 
> 
> On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > 
> > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > features to 0, when config space is accessed before features
> > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > and allow features reset to 0 for this case.
> > > > > > > 
> > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > config space before features are set. For instance, when
> > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > will try to access and validate the MTU present in the config
> > > > > > > space before virtio features are set.
> > > > > > This looks like a spec violation:
> > > > > > 
> > > > > > "
> > > > > > 
> > > > > > The following driver-read-only field, mtu only exists if
> > > > > > VIRTIO_NET_F_MTU is
> > > > > > set.
> > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > "
> > > > > > 
> > > > > > Do we really want to workaround this?
> > > > > > 
> > > > > > Thanks
> > > > > And also:
> > > > > 
> > > > > The driver MUST follow this sequence to initialize a device:
> > > > > 1. Reset the device.
> > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > device.
> > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > understood by the OS and driver to the
> > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > the device-specific configuration
> > > > > fields to check that it can support the device before accepting it.
> > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > feature bits after this step.
> > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > otherwise, the device does not
> > > > > support our subset of features and the device is unusable.
> > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > for the device, optional per-bus setup,
> > > > > reading and possibly writing the device’s virtio configuration
> > > > > space, and population of virtqueues.
> > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > 
> > > > > 
> > > > > so accessing config space before FEATURES_OK is a spec violation, right?
> > > > It is, but it's not relevant to what this commit tries to address. I
> > > > thought the legacy guest still needs to be supported.
> > > > 
> > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > issue where this discrepancy is introduced to virtnet_validate() (since
> > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > 
> > > > -Siwei
> > > 
> > > I think it's a bug to read config space in validate, we should move it to
> > > virtnet_probe().
> > > 
> > > Thanks
> > I take it back, reading but not writing seems to be explicitly allowed by spec.
> > So our way to detect a legacy guest is bogus, need to think what is
> > the best way to handle this.
> Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
> guest? Supposedly only config space write access needs to be guarded before
> setting FEATURES_OK.
> 
> -Siwie

Detecting it isn't enough though, we will need a new ioctl to notify
the kernel that it's a legacy guest. Ugh :(


> > > > > 
> > > > > > > Rejecting reset to 0
> > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > for the very first config space access, rendering issues like
> > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > out-of-range MTU.
> > > > > > > 
> > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > supported mlx5 devices")
> > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > ---
> > > > > > >     drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > >     1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > index 7c1f789..540dd67 100644
> > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > >         return mvdev->mlx_features;
> > > > > > >     }
> > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > u64 features)
> > > > > > > -{
> > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > -        return -EOPNOTSUPP;
> > > > > > > -
> > > > > > > -    return 0;
> > > > > > > -}
> > > > > > > -
> > > > > > >     static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > >     {
> > > > > > >         int err;
> > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > features)
> > > > > > >     {
> > > > > > >         struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > >         struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > -    int err;
> > > > > > >         print_features(mvdev, features, true);
> > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > -    if (err)
> > > > > > > -        return err;
> > > > > > > -
> > > > > > >         ndev->mvdev.actual_features = features &
> > > > > > > ndev->mvdev.mlx_features;
> > > > > > >         ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > >         ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > -    return err;
> > > > > > > +    return 0;
> > > > > > >     }
> > > > > > >     static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > *vdev, struct vdpa_callback *cb)

Michael S. Tsirkin Feb. 24, 2021, 5:17 a.m. UTC | #25

On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
> 
> On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
> > 
> > 
> > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > 
> > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > 
> > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > config space before features are set. For instance, when
> > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > space before virtio features are set.
> > > > > > > This looks like a spec violation:
> > > > > > > 
> > > > > > > "
> > > > > > > 
> > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > set.
> > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > "
> > > > > > > 
> > > > > > > Do we really want to workaround this?
> > > > > > > 
> > > > > > > Thanks
> > > > > > And also:
> > > > > > 
> > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > 1. Reset the device.
> > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has
> > > > > > noticed the device.
> > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > device.
> > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > understood by the OS and driver to the
> > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > the device-specific configuration
> > > > > > fields to check that it can support the device before accepting it.
> > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > feature bits after this step.
> > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > otherwise, the device does not
> > > > > > support our subset of features and the device is unusable.
> > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > for the device, optional per-bus setup,
> > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > space, and population of virtqueues.
> > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > 
> > > > > > 
> > > > > > so accessing config space before FEATURES_OK is a spec
> > > > > > violation, right?
> > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > thought the legacy guest still needs to be supported.
> > > > > 
> > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > issue where this discrepancy is introduced to
> > > > > virtnet_validate() (since
> > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > 
> > > > > -Siwei
> > > > 
> > > > I think it's a bug to read config space in validate, we should
> > > > move it to
> > > > virtnet_probe().
> > > > 
> > > > Thanks
> > > I take it back, reading but not writing seems to be explicitly
> > > allowed by spec.
> > > So our way to detect a legacy guest is bogus, need to think what is
> > > the best way to handle this.
> > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
> > legacy guest? Supposedly only config space write access needs to be
> > guarded before setting FEATURES_OK.
> 
> 
> I agree. My understanding is that all vDPA must be modern device (since
> VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
> 
> Thanks

Well mlx5 has some code to handle legacy guests ...
Eli, could you comment? Is that support unused right now?


> 
> > 
> > -Siwie
> > 
> > > > > > 
> > > > > > > > Rejecting reset to 0
> > > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > > for the very first config space access, rendering issues like
> > > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > > out-of-range MTU.
> > > > > > > > 
> > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > > supported mlx5 devices")
> > > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > > ---
> > > > > > > >     drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > > >     1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > index 7c1f789..540dd67 100644
> > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > > >         return mvdev->mlx_features;
> > > > > > > >     }
> > > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > > u64 features)
> > > > > > > > -{
> > > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > > -        return -EOPNOTSUPP;
> > > > > > > > -
> > > > > > > > -    return 0;
> > > > > > > > -}
> > > > > > > > -
> > > > > > > >     static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > > >     {
> > > > > > > >         int err;
> > > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > > features)
> > > > > > > >     {
> > > > > > > >         struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > > >         struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > > -    int err;
> > > > > > > >         print_features(mvdev, features, true);
> > > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > > -    if (err)
> > > > > > > > -        return err;
> > > > > > > > -
> > > > > > > >         ndev->mvdev.actual_features = features &
> > > > > > > > ndev->mvdev.mlx_features;
> > > > > > > >         ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > > >         ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > > -    return err;
> > > > > > > > +    return 0;
> > > > > > > >     }
> > > > > > > >     static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > > *vdev, struct vdpa_callback *cb)
> >

Jason Wang Feb. 24, 2021, 6:02 a.m. UTC | #26

On 2021/2/24 1:17 下午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
>> On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
>>>
>>> On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
>>>> On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
>>>>> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>>>>>> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>>>>>>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>>
>>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>>> config space before features are set. For instance, when
>>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>>> space before virtio features are set.
>>>>>>>> This looks like a spec violation:
>>>>>>>>
>>>>>>>> "
>>>>>>>>
>>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>>> VIRTIO_NET_F_MTU is
>>>>>>>> set.
>>>>>>>> This field specifies the maximum MTU for the driver to use.
>>>>>>>> "
>>>>>>>>
>>>>>>>> Do we really want to workaround this?
>>>>>>>>
>>>>>>>> Thanks
>>>>>>> And also:
>>>>>>>
>>>>>>> The driver MUST follow this sequence to initialize a device:
>>>>>>> 1. Reset the device.
>>>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has
>>>>>>> noticed the device.
>>>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the
>>>>>>> device.
>>>>>>> 4. Read device feature bits, and write the subset of feature bits
>>>>>>> understood by the OS and driver to the
>>>>>>> device. During this step the driver MAY read (but MUST NOT write)
>>>>>>> the device-specific configuration
>>>>>>> fields to check that it can support the device before accepting it.
>>>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
>>>>>>> feature bits after this step.
>>>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set:
>>>>>>> otherwise, the device does not
>>>>>>> support our subset of features and the device is unusable.
>>>>>>> 7. Perform device-specific setup, including discovery of virtqueues
>>>>>>> for the device, optional per-bus setup,
>>>>>>> reading and possibly writing the device’s virtio configuration
>>>>>>> space, and population of virtqueues.
>>>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>>>
>>>>>>>
>>>>>>> so accessing config space before FEATURES_OK is a spec
>>>>>>> violation, right?
>>>>>> It is, but it's not relevant to what this commit tries to address. I
>>>>>> thought the legacy guest still needs to be supported.
>>>>>>
>>>>>> Having said, a separate patch has to be posted to fix the guest driver
>>>>>> issue where this discrepancy is introduced to
>>>>>> virtnet_validate() (since
>>>>>> commit fe36cbe067). But it's not technically related to this patch.
>>>>>>
>>>>>> -Siwei
>>>>> I think it's a bug to read config space in validate, we should
>>>>> move it to
>>>>> virtnet_probe().
>>>>>
>>>>> Thanks
>>>> I take it back, reading but not writing seems to be explicitly
>>>> allowed by spec.
>>>> So our way to detect a legacy guest is bogus, need to think what is
>>>> the best way to handle this.
>>> Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
>>> legacy guest? Supposedly only config space write access needs to be
>>> guarded before setting FEATURES_OK.
>>
>> I agree. My understanding is that all vDPA must be modern device (since
>> VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
>>
>> Thanks
> Well mlx5 has some code to handle legacy guests ...


My understanding is that, even if mlx5 is modern device it can still 
suppot legacy guests since the device saw by guest is emulated by Qemu. 
Qemu can just present a transitional device to guest, but negotiate 
VIRTIO_F_ACCESS_PLATFORM. (Actually this is what has been done now).

Thanks


> Eli, could you comment? Is that support unused right now?
>
>
>>> -Siwie
>>>
>>>>>>>>> Rejecting reset to 0
>>>>>>>>> prematurely causes correct MTU and link status unable to load
>>>>>>>>> for the very first config space access, rendering issues like
>>>>>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>>>>>> out-of-range MTU.
>>>>>>>>>
>>>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
>>>>>>>>> supported mlx5 devices")
>>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>>>> ---
>>>>>>>>>      drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>>>>>      1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>> index 7c1f789..540dd67 100644
>>>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>> @@ -1490,14 +1490,6 @@ static u64
>>>>>>>>> mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>>>>>          return mvdev->mlx_features;
>>>>>>>>>      }
>>>>>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
>>>>>>>>> u64 features)
>>>>>>>>> -{
>>>>>>>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>>>>>> -        return -EOPNOTSUPP;
>>>>>>>>> -
>>>>>>>>> -    return 0;
>>>>>>>>> -}
>>>>>>>>> -
>>>>>>>>>      static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>>>>>>      {
>>>>>>>>>          int err;
>>>>>>>>> @@ -1558,18 +1550,13 @@ static int
>>>>>>>>> mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
>>>>>>>>> features)
>>>>>>>>>      {
>>>>>>>>>          struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>>>>>>          struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>>>>>>> -    int err;
>>>>>>>>>          print_features(mvdev, features, true);
>>>>>>>>> -    err = verify_min_features(mvdev, features);
>>>>>>>>> -    if (err)
>>>>>>>>> -        return err;
>>>>>>>>> -
>>>>>>>>>          ndev->mvdev.actual_features = features &
>>>>>>>>> ndev->mvdev.mlx_features;
>>>>>>>>>          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>>>>>>          ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
>>>>>>>>> VIRTIO_NET_S_LINK_UP);
>>>>>>>>> -    return err;
>>>>>>>>> +    return 0;
>>>>>>>>>      }
>>>>>>>>>      static void mlx5_vdpa_set_config_cb(struct vdpa_device
>>>>>>>>> *vdev, struct vdpa_callback *cb)

Jason Wang Feb. 24, 2021, 6:04 a.m. UTC | #27

On 2021/2/24 1:04 下午, Michael S. Tsirkin wrote:
> On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
>>
>> On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
>>> On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
>>>> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>>>>> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>>>>>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>
>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>> config space before features are set. For instance, when
>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>> space before virtio features are set.
>>>>>>> This looks like a spec violation:
>>>>>>>
>>>>>>> "
>>>>>>>
>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>> VIRTIO_NET_F_MTU is
>>>>>>> set.
>>>>>>> This field specifies the maximum MTU for the driver to use.
>>>>>>> "
>>>>>>>
>>>>>>> Do we really want to workaround this?
>>>>>>>
>>>>>>> Thanks
>>>>>> And also:
>>>>>>
>>>>>> The driver MUST follow this sequence to initialize a device:
>>>>>> 1. Reset the device.
>>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the
>>>>>> device.
>>>>>> 4. Read device feature bits, and write the subset of feature bits
>>>>>> understood by the OS and driver to the
>>>>>> device. During this step the driver MAY read (but MUST NOT write)
>>>>>> the device-specific configuration
>>>>>> fields to check that it can support the device before accepting it.
>>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
>>>>>> feature bits after this step.
>>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set:
>>>>>> otherwise, the device does not
>>>>>> support our subset of features and the device is unusable.
>>>>>> 7. Perform device-specific setup, including discovery of virtqueues
>>>>>> for the device, optional per-bus setup,
>>>>>> reading and possibly writing the device’s virtio configuration
>>>>>> space, and population of virtqueues.
>>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>>
>>>>>>
>>>>>> so accessing config space before FEATURES_OK is a spec violation, right?
>>>>> It is, but it's not relevant to what this commit tries to address. I
>>>>> thought the legacy guest still needs to be supported.
>>>>>
>>>>> Having said, a separate patch has to be posted to fix the guest driver
>>>>> issue where this discrepancy is introduced to virtnet_validate() (since
>>>>> commit fe36cbe067). But it's not technically related to this patch.
>>>>>
>>>>> -Siwei
>>>> I think it's a bug to read config space in validate, we should move it to
>>>> virtnet_probe().
>>>>
>>>> Thanks
>>> I take it back, reading but not writing seems to be explicitly allowed by spec.
>>> So our way to detect a legacy guest is bogus, need to think what is
>>> the best way to handle this.
>> Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
>> guest? Supposedly only config space write access needs to be guarded before
>> setting FEATURES_OK.
>>
>> -Siwie
> Detecting it isn't enough though, we will need a new ioctl to notify
> the kernel that it's a legacy guest. Ugh :(


I'm not sure I get this, how can we know if there's a legacy driver 
before set_features()?

And I wonder what will hapeen if we just revert the set_features(0)?

Thanks


>
>
>>>>>>>> Rejecting reset to 0
>>>>>>>> prematurely causes correct MTU and link status unable to load
>>>>>>>> for the very first config space access, rendering issues like
>>>>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>>>>> out-of-range MTU.
>>>>>>>>
>>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
>>>>>>>> supported mlx5 devices")
>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>>> ---
>>>>>>>>      drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>>>>      1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>> index 7c1f789..540dd67 100644
>>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>> @@ -1490,14 +1490,6 @@ static u64
>>>>>>>> mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>>>>          return mvdev->mlx_features;
>>>>>>>>      }
>>>>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
>>>>>>>> u64 features)
>>>>>>>> -{
>>>>>>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>>>>> -        return -EOPNOTSUPP;
>>>>>>>> -
>>>>>>>> -    return 0;
>>>>>>>> -}
>>>>>>>> -
>>>>>>>>      static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>>>>>      {
>>>>>>>>          int err;
>>>>>>>> @@ -1558,18 +1550,13 @@ static int
>>>>>>>> mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
>>>>>>>> features)
>>>>>>>>      {
>>>>>>>>          struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>>>>>          struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>>>>>> -    int err;
>>>>>>>>          print_features(mvdev, features, true);
>>>>>>>> -    err = verify_min_features(mvdev, features);
>>>>>>>> -    if (err)
>>>>>>>> -        return err;
>>>>>>>> -
>>>>>>>>          ndev->mvdev.actual_features = features &
>>>>>>>> ndev->mvdev.mlx_features;
>>>>>>>>          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>>>>>          ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
>>>>>>>> VIRTIO_NET_S_LINK_UP);
>>>>>>>> -    return err;
>>>>>>>> +    return 0;
>>>>>>>>      }
>>>>>>>>      static void mlx5_vdpa_set_config_cb(struct vdpa_device
>>>>>>>> *vdev, struct vdpa_callback *cb)

Eli Cohen Feb. 24, 2021, 6:45 a.m. UTC | #28

On Wed, Feb 24, 2021 at 12:17:58AM -0500, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
> > 
> > On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
> > > 
> > > 
> > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > 
> > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > 
> > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > space before virtio features are set.
> > > > > > > > This looks like a spec violation:
> > > > > > > > 
> > > > > > > > "
> > > > > > > > 
> > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > set.
> > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > "
> > > > > > > > 
> > > > > > > > Do we really want to workaround this?
> > > > > > > > 
> > > > > > > > Thanks
> > > > > > > And also:
> > > > > > > 
> > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > 1. Reset the device.
> > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has
> > > > > > > noticed the device.
> > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > device.
> > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > understood by the OS and driver to the
> > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > the device-specific configuration
> > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > feature bits after this step.
> > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > otherwise, the device does not
> > > > > > > support our subset of features and the device is unusable.
> > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > for the device, optional per-bus setup,
> > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > space, and population of virtqueues.
> > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > 
> > > > > > > 
> > > > > > > so accessing config space before FEATURES_OK is a spec
> > > > > > > violation, right?
> > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > thought the legacy guest still needs to be supported.
> > > > > > 
> > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > issue where this discrepancy is introduced to
> > > > > > virtnet_validate() (since
> > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > 
> > > > > > -Siwei
> > > > > 
> > > > > I think it's a bug to read config space in validate, we should
> > > > > move it to
> > > > > virtnet_probe().
> > > > > 
> > > > > Thanks
> > > > I take it back, reading but not writing seems to be explicitly
> > > > allowed by spec.
> > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > the best way to handle this.
> > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
> > > legacy guest? Supposedly only config space write access needs to be
> > > guarded before setting FEATURES_OK.
> > 
> > 
> > I agree. My understanding is that all vDPA must be modern device (since
> > VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
> > 
> > Thanks
> 
> Well mlx5 has some code to handle legacy guests ...
> Eli, could you comment? Is that support unused right now?
> 

If you mean support for version 1.0, well the knob is there but it's not
set in the firmware I use. Note sure if we will support this.

> 
> > 
> > > 
> > > -Siwie
> > > 
> > > > > > > 
> > > > > > > > > Rejecting reset to 0
> > > > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > > > for the very first config space access, rendering issues like
> > > > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > > > out-of-range MTU.
> > > > > > > > > 
> > > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > > > supported mlx5 devices")
> > > > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > > > ---
> > > > > > > > >     drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > > > >     1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > > > 
> > > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > index 7c1f789..540dd67 100644
> > > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > > > >         return mvdev->mlx_features;
> > > > > > > > >     }
> > > > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > > > u64 features)
> > > > > > > > > -{
> > > > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > > > -        return -EOPNOTSUPP;
> > > > > > > > > -
> > > > > > > > > -    return 0;
> > > > > > > > > -}
> > > > > > > > > -
> > > > > > > > >     static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > > > >     {
> > > > > > > > >         int err;
> > > > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > > > features)
> > > > > > > > >     {
> > > > > > > > >         struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > > > >         struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > > > -    int err;
> > > > > > > > >         print_features(mvdev, features, true);
> > > > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > > > -    if (err)
> > > > > > > > > -        return err;
> > > > > > > > > -
> > > > > > > > >         ndev->mvdev.actual_features = features &
> > > > > > > > > ndev->mvdev.mlx_features;
> > > > > > > > >         ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > > > >         ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > > > -    return err;
> > > > > > > > > +    return 0;
> > > > > > > > >     }
> > > > > > > > >     static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > > > *vdev, struct vdpa_callback *cb)
> > > 
>

Michael S. Tsirkin Feb. 24, 2021, 6:46 a.m. UTC | #29

On Wed, Feb 24, 2021 at 02:04:36PM +0800, Jason Wang wrote:
> 
> On 2021/2/24 1:04 下午, Michael S. Tsirkin wrote:
> > On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
> > > 
> > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > 
> > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > space before virtio features are set.
> > > > > > > > This looks like a spec violation:
> > > > > > > > 
> > > > > > > > "
> > > > > > > > 
> > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > set.
> > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > "
> > > > > > > > 
> > > > > > > > Do we really want to workaround this?
> > > > > > > > 
> > > > > > > > Thanks
> > > > > > > And also:
> > > > > > > 
> > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > 1. Reset the device.
> > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > device.
> > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > understood by the OS and driver to the
> > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > the device-specific configuration
> > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > feature bits after this step.
> > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > otherwise, the device does not
> > > > > > > support our subset of features and the device is unusable.
> > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > for the device, optional per-bus setup,
> > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > space, and population of virtqueues.
> > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > 
> > > > > > > 
> > > > > > > so accessing config space before FEATURES_OK is a spec violation, right?
> > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > thought the legacy guest still needs to be supported.
> > > > > > 
> > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > issue where this discrepancy is introduced to virtnet_validate() (since
> > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > 
> > > > > > -Siwei
> > > > > I think it's a bug to read config space in validate, we should move it to
> > > > > virtnet_probe().
> > > > > 
> > > > > Thanks
> > > > I take it back, reading but not writing seems to be explicitly allowed by spec.
> > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > the best way to handle this.
> > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
> > > guest? Supposedly only config space write access needs to be guarded before
> > > setting FEATURES_OK.
> > > 
> > > -Siwie
> > Detecting it isn't enough though, we will need a new ioctl to notify
> > the kernel that it's a legacy guest. Ugh :(
> 
> 
> I'm not sure I get this, how can we know if there's a legacy driver before
> set_features()?

qemu knows for sure. It does not communicate this information to the
kernel right now unfortunately.

> And I wonder what will hapeen if we just revert the set_features(0)?
> 
> Thanks
> 
> 
> > 
> > 
> > > > > > > > > Rejecting reset to 0
> > > > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > > > for the very first config space access, rendering issues like
> > > > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > > > out-of-range MTU.
> > > > > > > > > 
> > > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > > > supported mlx5 devices")
> > > > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > > > ---
> > > > > > > > >      drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > > > >      1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > > > 
> > > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > index 7c1f789..540dd67 100644
> > > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > > > >          return mvdev->mlx_features;
> > > > > > > > >      }
> > > > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > > > u64 features)
> > > > > > > > > -{
> > > > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > > > -        return -EOPNOTSUPP;
> > > > > > > > > -
> > > > > > > > > -    return 0;
> > > > > > > > > -}
> > > > > > > > > -
> > > > > > > > >      static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > > > >      {
> > > > > > > > >          int err;
> > > > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > > > features)
> > > > > > > > >      {
> > > > > > > > >          struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > > > >          struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > > > -    int err;
> > > > > > > > >          print_features(mvdev, features, true);
> > > > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > > > -    if (err)
> > > > > > > > > -        return err;
> > > > > > > > > -
> > > > > > > > >          ndev->mvdev.actual_features = features &
> > > > > > > > > ndev->mvdev.mlx_features;
> > > > > > > > >          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > > > >          ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > > > -    return err;
> > > > > > > > > +    return 0;
> > > > > > > > >      }
> > > > > > > > >      static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > > > *vdev, struct vdpa_callback *cb)

Michael S. Tsirkin Feb. 24, 2021, 6:47 a.m. UTC | #30

On Wed, Feb 24, 2021 at 08:45:20AM +0200, Eli Cohen wrote:
> On Wed, Feb 24, 2021 at 12:17:58AM -0500, Michael S. Tsirkin wrote:
> > On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
> > > 
> > > On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
> > > > 
> > > > 
> > > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > > 
> > > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > > 
> > > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > > space before virtio features are set.
> > > > > > > > > This looks like a spec violation:
> > > > > > > > > 
> > > > > > > > > "
> > > > > > > > > 
> > > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > > set.
> > > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > > "
> > > > > > > > > 
> > > > > > > > > Do we really want to workaround this?
> > > > > > > > > 
> > > > > > > > > Thanks
> > > > > > > > And also:
> > > > > > > > 
> > > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > > 1. Reset the device.
> > > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has
> > > > > > > > noticed the device.
> > > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > > device.
> > > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > > understood by the OS and driver to the
> > > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > > the device-specific configuration
> > > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > > feature bits after this step.
> > > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > > otherwise, the device does not
> > > > > > > > support our subset of features and the device is unusable.
> > > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > > for the device, optional per-bus setup,
> > > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > > space, and population of virtqueues.
> > > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > > 
> > > > > > > > 
> > > > > > > > so accessing config space before FEATURES_OK is a spec
> > > > > > > > violation, right?
> > > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > > thought the legacy guest still needs to be supported.
> > > > > > > 
> > > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > > issue where this discrepancy is introduced to
> > > > > > > virtnet_validate() (since
> > > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > > 
> > > > > > > -Siwei
> > > > > > 
> > > > > > I think it's a bug to read config space in validate, we should
> > > > > > move it to
> > > > > > virtnet_probe().
> > > > > > 
> > > > > > Thanks
> > > > > I take it back, reading but not writing seems to be explicitly
> > > > > allowed by spec.
> > > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > > the best way to handle this.
> > > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
> > > > legacy guest? Supposedly only config space write access needs to be
> > > > guarded before setting FEATURES_OK.
> > > 
> > > 
> > > I agree. My understanding is that all vDPA must be modern device (since
> > > VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
> > > 
> > > Thanks
> > 
> > Well mlx5 has some code to handle legacy guests ...
> > Eli, could you comment? Is that support unused right now?
> > 
> 
> If you mean support for version 1.0, well the knob is there but it's not
> set in the firmware I use. Note sure if we will support this.

Hmm you mean it's legacy only right now?
Well at some point you will want advanced goodies like RSS
and all that is gated on 1.0 ;)

> > 
> > > 
> > > > 
> > > > -Siwie
> > > > 
> > > > > > > > 
> > > > > > > > > > Rejecting reset to 0
> > > > > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > > > > for the very first config space access, rendering issues like
> > > > > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > > > > out-of-range MTU.
> > > > > > > > > > 
> > > > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > > > > supported mlx5 devices")
> > > > > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > > > > ---
> > > > > > > > > >     drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > > > > >     1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > > > > 
> > > > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > index 7c1f789..540dd67 100644
> > > > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > > > > >         return mvdev->mlx_features;
> > > > > > > > > >     }
> > > > > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > > > > u64 features)
> > > > > > > > > > -{
> > > > > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > > > > -        return -EOPNOTSUPP;
> > > > > > > > > > -
> > > > > > > > > > -    return 0;
> > > > > > > > > > -}
> > > > > > > > > > -
> > > > > > > > > >     static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > > > > >     {
> > > > > > > > > >         int err;
> > > > > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > > > > features)
> > > > > > > > > >     {
> > > > > > > > > >         struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > > > > >         struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > > > > -    int err;
> > > > > > > > > >         print_features(mvdev, features, true);
> > > > > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > > > > -    if (err)
> > > > > > > > > > -        return err;
> > > > > > > > > > -
> > > > > > > > > >         ndev->mvdev.actual_features = features &
> > > > > > > > > > ndev->mvdev.mlx_features;
> > > > > > > > > >         ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > > > > >         ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > > > > -    return err;
> > > > > > > > > > +    return 0;
> > > > > > > > > >     }
> > > > > > > > > >     static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > > > > *vdev, struct vdpa_callback *cb)
> > > > 
> >

Jason Wang Feb. 24, 2021, 6:53 a.m. UTC | #31

On 2021/2/24 2:46 下午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 02:04:36PM +0800, Jason Wang wrote:
>> On 2021/2/24 1:04 下午, Michael S. Tsirkin wrote:
>>> On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
>>>> On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
>>>>> On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
>>>>>> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>>>>>>> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>>>>>>>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>>>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>>>
>>>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>>>> config space before features are set. For instance, when
>>>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>>>> space before virtio features are set.
>>>>>>>>> This looks like a spec violation:
>>>>>>>>>
>>>>>>>>> "
>>>>>>>>>
>>>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>>>> VIRTIO_NET_F_MTU is
>>>>>>>>> set.
>>>>>>>>> This field specifies the maximum MTU for the driver to use.
>>>>>>>>> "
>>>>>>>>>
>>>>>>>>> Do we really want to workaround this?
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>> And also:
>>>>>>>>
>>>>>>>> The driver MUST follow this sequence to initialize a device:
>>>>>>>> 1. Reset the device.
>>>>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>>>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the
>>>>>>>> device.
>>>>>>>> 4. Read device feature bits, and write the subset of feature bits
>>>>>>>> understood by the OS and driver to the
>>>>>>>> device. During this step the driver MAY read (but MUST NOT write)
>>>>>>>> the device-specific configuration
>>>>>>>> fields to check that it can support the device before accepting it.
>>>>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
>>>>>>>> feature bits after this step.
>>>>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set:
>>>>>>>> otherwise, the device does not
>>>>>>>> support our subset of features and the device is unusable.
>>>>>>>> 7. Perform device-specific setup, including discovery of virtqueues
>>>>>>>> for the device, optional per-bus setup,
>>>>>>>> reading and possibly writing the device’s virtio configuration
>>>>>>>> space, and population of virtqueues.
>>>>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>>>>
>>>>>>>>
>>>>>>>> so accessing config space before FEATURES_OK is a spec violation, right?
>>>>>>> It is, but it's not relevant to what this commit tries to address. I
>>>>>>> thought the legacy guest still needs to be supported.
>>>>>>>
>>>>>>> Having said, a separate patch has to be posted to fix the guest driver
>>>>>>> issue where this discrepancy is introduced to virtnet_validate() (since
>>>>>>> commit fe36cbe067). But it's not technically related to this patch.
>>>>>>>
>>>>>>> -Siwei
>>>>>> I think it's a bug to read config space in validate, we should move it to
>>>>>> virtnet_probe().
>>>>>>
>>>>>> Thanks
>>>>> I take it back, reading but not writing seems to be explicitly allowed by spec.
>>>>> So our way to detect a legacy guest is bogus, need to think what is
>>>>> the best way to handle this.
>>>> Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
>>>> guest? Supposedly only config space write access needs to be guarded before
>>>> setting FEATURES_OK.
>>>>
>>>> -Siwie
>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>> the kernel that it's a legacy guest. Ugh :(
>>
>> I'm not sure I get this, how can we know if there's a legacy driver before
>> set_features()?
> qemu knows for sure. It does not communicate this information to the
> kernel right now unfortunately.


I may miss something, but I still don't get how the new ioctl is 
supposed to work.

Thanks


>
>> And I wonder what will hapeen if we just revert the set_features(0)?
>>
>> Thanks
>>
>>
>>>
>>>>>>>>>> Rejecting reset to 0
>>>>>>>>>> prematurely causes correct MTU and link status unable to load
>>>>>>>>>> for the very first config space access, rendering issues like
>>>>>>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>>>>>>> out-of-range MTU.
>>>>>>>>>>
>>>>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
>>>>>>>>>> supported mlx5 devices")
>>>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>>>>> ---
>>>>>>>>>>       drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>>>>>>       1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>> index 7c1f789..540dd67 100644
>>>>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>> @@ -1490,14 +1490,6 @@ static u64
>>>>>>>>>> mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>>>>>>           return mvdev->mlx_features;
>>>>>>>>>>       }
>>>>>>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
>>>>>>>>>> u64 features)
>>>>>>>>>> -{
>>>>>>>>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>>>>>>> -        return -EOPNOTSUPP;
>>>>>>>>>> -
>>>>>>>>>> -    return 0;
>>>>>>>>>> -}
>>>>>>>>>> -
>>>>>>>>>>       static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>>>>>>>       {
>>>>>>>>>>           int err;
>>>>>>>>>> @@ -1558,18 +1550,13 @@ static int
>>>>>>>>>> mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
>>>>>>>>>> features)
>>>>>>>>>>       {
>>>>>>>>>>           struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>>>>>>>           struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>>>>>>>> -    int err;
>>>>>>>>>>           print_features(mvdev, features, true);
>>>>>>>>>> -    err = verify_min_features(mvdev, features);
>>>>>>>>>> -    if (err)
>>>>>>>>>> -        return err;
>>>>>>>>>> -
>>>>>>>>>>           ndev->mvdev.actual_features = features &
>>>>>>>>>> ndev->mvdev.mlx_features;
>>>>>>>>>>           ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>>>>>>>           ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
>>>>>>>>>> VIRTIO_NET_S_LINK_UP);
>>>>>>>>>> -    return err;
>>>>>>>>>> +    return 0;
>>>>>>>>>>       }
>>>>>>>>>>       static void mlx5_vdpa_set_config_cb(struct vdpa_device
>>>>>>>>>> *vdev, struct vdpa_callback *cb)

Jason Wang Feb. 24, 2021, 6:55 a.m. UTC | #32

On 2021/2/24 2:47 下午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 08:45:20AM +0200, Eli Cohen wrote:
>> On Wed, Feb 24, 2021 at 12:17:58AM -0500, Michael S. Tsirkin wrote:
>>> On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
>>>> On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
>>>>>
>>>>> On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
>>>>>> On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
>>>>>>> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>>>>>>>> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>>>>>>>>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>>>>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>>>>
>>>>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>>>>> config space before features are set. For instance, when
>>>>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>>>>> space before virtio features are set.
>>>>>>>>>> This looks like a spec violation:
>>>>>>>>>>
>>>>>>>>>> "
>>>>>>>>>>
>>>>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>>>>> VIRTIO_NET_F_MTU is
>>>>>>>>>> set.
>>>>>>>>>> This field specifies the maximum MTU for the driver to use.
>>>>>>>>>> "
>>>>>>>>>>
>>>>>>>>>> Do we really want to workaround this?
>>>>>>>>>>
>>>>>>>>>> Thanks
>>>>>>>>> And also:
>>>>>>>>>
>>>>>>>>> The driver MUST follow this sequence to initialize a device:
>>>>>>>>> 1. Reset the device.
>>>>>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has
>>>>>>>>> noticed the device.
>>>>>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the
>>>>>>>>> device.
>>>>>>>>> 4. Read device feature bits, and write the subset of feature bits
>>>>>>>>> understood by the OS and driver to the
>>>>>>>>> device. During this step the driver MAY read (but MUST NOT write)
>>>>>>>>> the device-specific configuration
>>>>>>>>> fields to check that it can support the device before accepting it.
>>>>>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
>>>>>>>>> feature bits after this step.
>>>>>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set:
>>>>>>>>> otherwise, the device does not
>>>>>>>>> support our subset of features and the device is unusable.
>>>>>>>>> 7. Perform device-specific setup, including discovery of virtqueues
>>>>>>>>> for the device, optional per-bus setup,
>>>>>>>>> reading and possibly writing the device’s virtio configuration
>>>>>>>>> space, and population of virtqueues.
>>>>>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> so accessing config space before FEATURES_OK is a spec
>>>>>>>>> violation, right?
>>>>>>>> It is, but it's not relevant to what this commit tries to address. I
>>>>>>>> thought the legacy guest still needs to be supported.
>>>>>>>>
>>>>>>>> Having said, a separate patch has to be posted to fix the guest driver
>>>>>>>> issue where this discrepancy is introduced to
>>>>>>>> virtnet_validate() (since
>>>>>>>> commit fe36cbe067). But it's not technically related to this patch.
>>>>>>>>
>>>>>>>> -Siwei
>>>>>>> I think it's a bug to read config space in validate, we should
>>>>>>> move it to
>>>>>>> virtnet_probe().
>>>>>>>
>>>>>>> Thanks
>>>>>> I take it back, reading but not writing seems to be explicitly
>>>>>> allowed by spec.
>>>>>> So our way to detect a legacy guest is bogus, need to think what is
>>>>>> the best way to handle this.
>>>>> Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
>>>>> legacy guest? Supposedly only config space write access needs to be
>>>>> guarded before setting FEATURES_OK.
>>>>
>>>> I agree. My understanding is that all vDPA must be modern device (since
>>>> VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
>>>>
>>>> Thanks
>>> Well mlx5 has some code to handle legacy guests ...
>>> Eli, could you comment? Is that support unused right now?
>>>
>> If you mean support for version 1.0, well the knob is there but it's not
>> set in the firmware I use. Note sure if we will support this.
> Hmm you mean it's legacy only right now?
> Well at some point you will want advanced goodies like RSS
> and all that is gated on 1.0 ;)


So if my understanding is correct the device/firmware is legacy but 
require VIRTIO_F_ACCESS_PLATFORM semanic? Looks like a spec violation?

Thanks


>
>>>>> -Siwie
>>>>>
>>>>>>>>>>> Rejecting reset to 0
>>>>>>>>>>> prematurely causes correct MTU and link status unable to load
>>>>>>>>>>> for the very first config space access, rendering issues like
>>>>>>>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>>>>>>>> out-of-range MTU.
>>>>>>>>>>>
>>>>>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
>>>>>>>>>>> supported mlx5 devices")
>>>>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>>>>>> ---
>>>>>>>>>>>      drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>>>>>>>      1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>>>>>>>
>>>>>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>>> index 7c1f789..540dd67 100644
>>>>>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>>>>> @@ -1490,14 +1490,6 @@ static u64
>>>>>>>>>>> mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>>>>>>>          return mvdev->mlx_features;
>>>>>>>>>>>      }
>>>>>>>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
>>>>>>>>>>> u64 features)
>>>>>>>>>>> -{
>>>>>>>>>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>>>>>>>> -        return -EOPNOTSUPP;
>>>>>>>>>>> -
>>>>>>>>>>> -    return 0;
>>>>>>>>>>> -}
>>>>>>>>>>> -
>>>>>>>>>>>      static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>>>>>>>>      {
>>>>>>>>>>>          int err;
>>>>>>>>>>> @@ -1558,18 +1550,13 @@ static int
>>>>>>>>>>> mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
>>>>>>>>>>> features)
>>>>>>>>>>>      {
>>>>>>>>>>>          struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>>>>>>>>          struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>>>>>>>>> -    int err;
>>>>>>>>>>>          print_features(mvdev, features, true);
>>>>>>>>>>> -    err = verify_min_features(mvdev, features);
>>>>>>>>>>> -    if (err)
>>>>>>>>>>> -        return err;
>>>>>>>>>>> -
>>>>>>>>>>>          ndev->mvdev.actual_features = features &
>>>>>>>>>>> ndev->mvdev.mlx_features;
>>>>>>>>>>>          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>>>>>>>>          ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
>>>>>>>>>>> VIRTIO_NET_S_LINK_UP);
>>>>>>>>>>> -    return err;
>>>>>>>>>>> +    return 0;
>>>>>>>>>>>      }
>>>>>>>>>>>      static void mlx5_vdpa_set_config_cb(struct vdpa_device
>>>>>>>>>>> *vdev, struct vdpa_callback *cb)

Michael S. Tsirkin Feb. 24, 2021, 7:12 a.m. UTC | #33

On Wed, Feb 24, 2021 at 02:55:13PM +0800, Jason Wang wrote:
> 
> On 2021/2/24 2:47 下午, Michael S. Tsirkin wrote:
> > On Wed, Feb 24, 2021 at 08:45:20AM +0200, Eli Cohen wrote:
> > > On Wed, Feb 24, 2021 at 12:17:58AM -0500, Michael S. Tsirkin wrote:
> > > > On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
> > > > > On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
> > > > > > 
> > > > > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > > > > 
> > > > > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > > > > space before virtio features are set.
> > > > > > > > > > > This looks like a spec violation:
> > > > > > > > > > > 
> > > > > > > > > > > "
> > > > > > > > > > > 
> > > > > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > > > > set.
> > > > > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > > > > "
> > > > > > > > > > > 
> > > > > > > > > > > Do we really want to workaround this?
> > > > > > > > > > > 
> > > > > > > > > > > Thanks
> > > > > > > > > > And also:
> > > > > > > > > > 
> > > > > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > > > > 1. Reset the device.
> > > > > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has
> > > > > > > > > > noticed the device.
> > > > > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > > > > device.
> > > > > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > > > > understood by the OS and driver to the
> > > > > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > > > > the device-specific configuration
> > > > > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > > > > feature bits after this step.
> > > > > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > > > > otherwise, the device does not
> > > > > > > > > > support our subset of features and the device is unusable.
> > > > > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > > > > for the device, optional per-bus setup,
> > > > > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > > > > space, and population of virtqueues.
> > > > > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > so accessing config space before FEATURES_OK is a spec
> > > > > > > > > > violation, right?
> > > > > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > > > > thought the legacy guest still needs to be supported.
> > > > > > > > > 
> > > > > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > > > > issue where this discrepancy is introduced to
> > > > > > > > > virtnet_validate() (since
> > > > > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > > > > 
> > > > > > > > > -Siwei
> > > > > > > > I think it's a bug to read config space in validate, we should
> > > > > > > > move it to
> > > > > > > > virtnet_probe().
> > > > > > > > 
> > > > > > > > Thanks
> > > > > > > I take it back, reading but not writing seems to be explicitly
> > > > > > > allowed by spec.
> > > > > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > > > > the best way to handle this.
> > > > > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
> > > > > > legacy guest? Supposedly only config space write access needs to be
> > > > > > guarded before setting FEATURES_OK.
> > > > > 
> > > > > I agree. My understanding is that all vDPA must be modern device (since
> > > > > VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
> > > > > 
> > > > > Thanks
> > > > Well mlx5 has some code to handle legacy guests ...
> > > > Eli, could you comment? Is that support unused right now?
> > > > 
> > > If you mean support for version 1.0, well the knob is there but it's not
> > > set in the firmware I use. Note sure if we will support this.
> > Hmm you mean it's legacy only right now?
> > Well at some point you will want advanced goodies like RSS
> > and all that is gated on 1.0 ;)
> 
> 
> So if my understanding is correct the device/firmware is legacy but require
> VIRTIO_F_ACCESS_PLATFORM semanic? Looks like a spec violation?
> 
> Thanks

Legacy mode description is the spec is non-normative. As such as long as
guests work, they work ;)

> 
> > 
> > > > > > -Siwie
> > > > > > 
> > > > > > > > > > > > Rejecting reset to 0
> > > > > > > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > > > > > > for the very first config space access, rendering issues like
> > > > > > > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > > > > > > out-of-range MTU.
> > > > > > > > > > > > 
> > > > > > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > > > > > > supported mlx5 devices")
> > > > > > > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > > > > > > ---
> > > > > > > > > > > >      drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > > > > > > >      1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > > > > > > 
> > > > > > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > index 7c1f789..540dd67 100644
> > > > > > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > > > > > > >          return mvdev->mlx_features;
> > > > > > > > > > > >      }
> > > > > > > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > > > > > > u64 features)
> > > > > > > > > > > > -{
> > > > > > > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > > > > > > -        return -EOPNOTSUPP;
> > > > > > > > > > > > -
> > > > > > > > > > > > -    return 0;
> > > > > > > > > > > > -}
> > > > > > > > > > > > -
> > > > > > > > > > > >      static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > > > > > > >      {
> > > > > > > > > > > >          int err;
> > > > > > > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > > > > > > features)
> > > > > > > > > > > >      {
> > > > > > > > > > > >          struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > > > > > > >          struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > > > > > > -    int err;
> > > > > > > > > > > >          print_features(mvdev, features, true);
> > > > > > > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > > > > > > -    if (err)
> > > > > > > > > > > > -        return err;
> > > > > > > > > > > > -
> > > > > > > > > > > >          ndev->mvdev.actual_features = features &
> > > > > > > > > > > > ndev->mvdev.mlx_features;
> > > > > > > > > > > >          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > > > > > > >          ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > > > > > > -    return err;
> > > > > > > > > > > > +    return 0;
> > > > > > > > > > > >      }
> > > > > > > > > > > >      static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > > > > > > *vdev, struct vdpa_callback *cb)

Eli Cohen Feb. 24, 2021, 7:17 a.m. UTC | #34

On Wed, Feb 24, 2021 at 02:55:13PM +0800, Jason Wang wrote:
> 
> On 2021/2/24 2:47 下午, Michael S. Tsirkin wrote:
> > On Wed, Feb 24, 2021 at 08:45:20AM +0200, Eli Cohen wrote:
> > > On Wed, Feb 24, 2021 at 12:17:58AM -0500, Michael S. Tsirkin wrote:
> > > > On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
> > > > > On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
> > > > > > 
> > > > > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > > > > 
> > > > > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > > > > space before virtio features are set.
> > > > > > > > > > > This looks like a spec violation:
> > > > > > > > > > > 
> > > > > > > > > > > "
> > > > > > > > > > > 
> > > > > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > > > > set.
> > > > > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > > > > "
> > > > > > > > > > > 
> > > > > > > > > > > Do we really want to workaround this?
> > > > > > > > > > > 
> > > > > > > > > > > Thanks
> > > > > > > > > > And also:
> > > > > > > > > > 
> > > > > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > > > > 1. Reset the device.
> > > > > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has
> > > > > > > > > > noticed the device.
> > > > > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > > > > device.
> > > > > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > > > > understood by the OS and driver to the
> > > > > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > > > > the device-specific configuration
> > > > > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > > > > feature bits after this step.
> > > > > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > > > > otherwise, the device does not
> > > > > > > > > > support our subset of features and the device is unusable.
> > > > > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > > > > for the device, optional per-bus setup,
> > > > > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > > > > space, and population of virtqueues.
> > > > > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > so accessing config space before FEATURES_OK is a spec
> > > > > > > > > > violation, right?
> > > > > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > > > > thought the legacy guest still needs to be supported.
> > > > > > > > > 
> > > > > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > > > > issue where this discrepancy is introduced to
> > > > > > > > > virtnet_validate() (since
> > > > > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > > > > 
> > > > > > > > > -Siwei
> > > > > > > > I think it's a bug to read config space in validate, we should
> > > > > > > > move it to
> > > > > > > > virtnet_probe().
> > > > > > > > 
> > > > > > > > Thanks
> > > > > > > I take it back, reading but not writing seems to be explicitly
> > > > > > > allowed by spec.
> > > > > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > > > > the best way to handle this.
> > > > > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
> > > > > > legacy guest? Supposedly only config space write access needs to be
> > > > > > guarded before setting FEATURES_OK.
> > > > > 
> > > > > I agree. My understanding is that all vDPA must be modern device (since
> > > > > VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
> > > > > 
> > > > > Thanks
> > > > Well mlx5 has some code to handle legacy guests ...
> > > > Eli, could you comment? Is that support unused right now?
> > > > 
> > > If you mean support for version 1.0, well the knob is there but it's not
> > > set in the firmware I use. Note sure if we will support this.
> > Hmm you mean it's legacy only right now?
> > Well at some point you will want advanced goodies like RSS
> > and all that is gated on 1.0 ;)
> 
> 
> So if my understanding is correct the device/firmware is legacy but require
> VIRTIO_F_ACCESS_PLATFORM semanic? Looks like a spec violation?
> 

I am checking this with some folks here.

> Thanks
> 
> 
> > 
> > > > > > -Siwie
> > > > > > 
> > > > > > > > > > > > Rejecting reset to 0
> > > > > > > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > > > > > > for the very first config space access, rendering issues like
> > > > > > > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > > > > > > out-of-range MTU.
> > > > > > > > > > > > 
> > > > > > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > > > > > > supported mlx5 devices")
> > > > > > > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > > > > > > ---
> > > > > > > > > > > >      drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > > > > > > >      1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > > > > > > 
> > > > > > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > index 7c1f789..540dd67 100644
> > > > > > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > > > > > > >          return mvdev->mlx_features;
> > > > > > > > > > > >      }
> > > > > > > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > > > > > > u64 features)
> > > > > > > > > > > > -{
> > > > > > > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > > > > > > -        return -EOPNOTSUPP;
> > > > > > > > > > > > -
> > > > > > > > > > > > -    return 0;
> > > > > > > > > > > > -}
> > > > > > > > > > > > -
> > > > > > > > > > > >      static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > > > > > > >      {
> > > > > > > > > > > >          int err;
> > > > > > > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > > > > > > features)
> > > > > > > > > > > >      {
> > > > > > > > > > > >          struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > > > > > > >          struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > > > > > > -    int err;
> > > > > > > > > > > >          print_features(mvdev, features, true);
> > > > > > > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > > > > > > -    if (err)
> > > > > > > > > > > > -        return err;
> > > > > > > > > > > > -
> > > > > > > > > > > >          ndev->mvdev.actual_features = features &
> > > > > > > > > > > > ndev->mvdev.mlx_features;
> > > > > > > > > > > >          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > > > > > > >          ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > > > > > > -    return err;
> > > > > > > > > > > > +    return 0;
> > > > > > > > > > > >      }
> > > > > > > > > > > >      static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > > > > > > *vdev, struct vdpa_callback *cb)
>

Michael S. Tsirkin Feb. 24, 2021, 7:17 a.m. UTC | #35

On Wed, Feb 24, 2021 at 02:53:08PM +0800, Jason Wang wrote:
> 
> On 2021/2/24 2:46 下午, Michael S. Tsirkin wrote:
> > On Wed, Feb 24, 2021 at 02:04:36PM +0800, Jason Wang wrote:
> > > On 2021/2/24 1:04 下午, Michael S. Tsirkin wrote:
> > > > On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
> > > > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > > > 
> > > > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > > > space before virtio features are set.
> > > > > > > > > > This looks like a spec violation:
> > > > > > > > > > 
> > > > > > > > > > "
> > > > > > > > > > 
> > > > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > > > set.
> > > > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > > > "
> > > > > > > > > > 
> > > > > > > > > > Do we really want to workaround this?
> > > > > > > > > > 
> > > > > > > > > > Thanks
> > > > > > > > > And also:
> > > > > > > > > 
> > > > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > > > 1. Reset the device.
> > > > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > > > device.
> > > > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > > > understood by the OS and driver to the
> > > > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > > > the device-specific configuration
> > > > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > > > feature bits after this step.
> > > > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > > > otherwise, the device does not
> > > > > > > > > support our subset of features and the device is unusable.
> > > > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > > > for the device, optional per-bus setup,
> > > > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > > > space, and population of virtqueues.
> > > > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > > > 
> > > > > > > > > 
> > > > > > > > > so accessing config space before FEATURES_OK is a spec violation, right?
> > > > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > > > thought the legacy guest still needs to be supported.
> > > > > > > > 
> > > > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > > > issue where this discrepancy is introduced to virtnet_validate() (since
> > > > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > > > 
> > > > > > > > -Siwei
> > > > > > > I think it's a bug to read config space in validate, we should move it to
> > > > > > > virtnet_probe().
> > > > > > > 
> > > > > > > Thanks
> > > > > > I take it back, reading but not writing seems to be explicitly allowed by spec.
> > > > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > > > the best way to handle this.
> > > > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
> > > > > guest? Supposedly only config space write access needs to be guarded before
> > > > > setting FEATURES_OK.
> > > > > 
> > > > > -Siwie
> > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > the kernel that it's a legacy guest. Ugh :(
> > > 
> > > I'm not sure I get this, how can we know if there's a legacy driver before
> > > set_features()?
> > qemu knows for sure. It does not communicate this information to the
> > kernel right now unfortunately.
> 
> 
> I may miss something, but I still don't get how the new ioctl is supposed to
> work.
> 
> Thanks



Basically on first guest access QEMU would tell kernel whether
guest is using the legacy or the modern interface.
E.g. virtio_pci_config_read/virtio_pci_config_write will call ioctl(ENABLE_LEGACY, 1)
while virtio_pci_common_read will call ioctl(ENABLE_LEGACY, 0)

Or maybe we just add GET_CONFIG_MODERN and GET_CONFIG_LEGACY and
call the correct ioctl ... there are many ways to build this API.

> 
> > 
> > > And I wonder what will hapeen if we just revert the set_features(0)?
> > > 
> > > Thanks
> > > 
> > > 
> > > > 
> > > > > > > > > > > Rejecting reset to 0
> > > > > > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > > > > > for the very first config space access, rendering issues like
> > > > > > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > > > > > out-of-range MTU.
> > > > > > > > > > > 
> > > > > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > > > > > supported mlx5 devices")
> > > > > > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > > > > > ---
> > > > > > > > > > >       drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > > > > > >       1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > > > > > 
> > > > > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > index 7c1f789..540dd67 100644
> > > > > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > > > > > >           return mvdev->mlx_features;
> > > > > > > > > > >       }
> > > > > > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > > > > > u64 features)
> > > > > > > > > > > -{
> > > > > > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > > > > > -        return -EOPNOTSUPP;
> > > > > > > > > > > -
> > > > > > > > > > > -    return 0;
> > > > > > > > > > > -}
> > > > > > > > > > > -
> > > > > > > > > > >       static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > > > > > >       {
> > > > > > > > > > >           int err;
> > > > > > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > > > > > features)
> > > > > > > > > > >       {
> > > > > > > > > > >           struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > > > > > >           struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > > > > > -    int err;
> > > > > > > > > > >           print_features(mvdev, features, true);
> > > > > > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > > > > > -    if (err)
> > > > > > > > > > > -        return err;
> > > > > > > > > > > -
> > > > > > > > > > >           ndev->mvdev.actual_features = features &
> > > > > > > > > > > ndev->mvdev.mlx_features;
> > > > > > > > > > >           ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > > > > > >           ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > > > > > -    return err;
> > > > > > > > > > > +    return 0;
> > > > > > > > > > >       }
> > > > > > > > > > >       static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > > > > > *vdev, struct vdpa_callback *cb)

Michael S. Tsirkin Feb. 24, 2021, 8:43 a.m. UTC | #36

On Wed, Feb 24, 2021 at 04:26:43PM +0800, Jason Wang wrote:
>     Basically on first guest access QEMU would tell kernel whether
>     guest is using the legacy or the modern interface.
>     E.g. virtio_pci_config_read/virtio_pci_config_write will call ioctl(ENABLE_LEGACY, 1)
>     while virtio_pci_common_read will call ioctl(ENABLE_LEGACY, 0)
> 
> 
> But this trick work only for PCI I think?
> 
> Thanks

ccw has a revision it can check. mmio does not have transitional devices
at all.

Jason Wang Feb. 24, 2021, 9:29 a.m. UTC | #37

On 2021/2/23 6:58 下午, Cornelia Huck wrote:
> On Tue, 23 Feb 2021 18:31:07 +0800
> Jason Wang <[e-mail]> wrote:
>
>> On 2021/2/23 6:04 下午, Cornelia Huck wrote:
>>> On Tue, 23 Feb 2021 17:46:20 +0800
>>> Jason Wang <[e-mail]> wrote:
>>>   
>>>> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
>>>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
>>>>>> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>
>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>> config space before features are set. For instance, when
>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>> space before virtio features are set.
>>>>>>> This looks like a spec violation:
>>>>>>>
>>>>>>> "
>>>>>>>
>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
>>>>>>> driver to use.
>>>>>>> "
>>>>>>>
>>>>>>> Do we really want to workaround this?
>>>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
>>>>>>
>>>>>> I think the point is, since there's legacy guest we'd have to support, this
>>>>>> host side workaround is unavoidable. Although I agree the violating driver
>>>>>> should be fixed (yes, it's in today's upstream kernel which exists for a
>>>>>> while now).
>>>>> Oh  you are right:
>>>>>
>>>>>
>>>>> static int virtnet_validate(struct virtio_device *vdev)
>>>>> {
>>>>>            if (!vdev->config->get) {
>>>>>                    dev_err(&vdev->dev, "%s failure: config access disabled\n",
>>>>>                            __func__);
>>>>>                    return -EINVAL;
>>>>>            }
>>>>>
>>>>>            if (!virtnet_validate_features(vdev))
>>>>>                    return -EINVAL;
>>>>>
>>>>>            if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>>>>                    int mtu = virtio_cread16(vdev,
>>>>>                                             offsetof(struct virtio_net_config,
>>>>>                                                      mtu));
>>>>>                    if (mtu < MIN_MTU)
>>>>>                            __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>>> I wonder why not simply fail here?
>>> I think both failing or not accepting the feature can be argued to make
>>> sense: "the device presented us with a mtu size that does not make
>>> sense" would point to failing, "we cannot work with the mtu size that
>>> the device presented us" would point to not negotiating the feature.
>>>   
>>>>   
>>>>>            }
>>>>>
>>>>>            return 0;
>>>>> }
>>>>>
>>>>> And the spec says:
>>>>>
>>>>>
>>>>> The driver MUST follow this sequence to initialize a device:
>>>>> 1. Reset the device.
>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
>>>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
>>>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
>>>>> fields to check that it can support the device before accepting it.
>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
>>>>> support our subset of features and the device is unusable.
>>>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
>>>>> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>
>>>>>
>>>>> Item 4 on the list explicitly allows reading config space before
>>>>> FEATURES_OK.
>>>>>
>>>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
>>>> So this probably need some clarification. "is set" is used many times in
>>>> the spec that has different implications.
>>> Before FEATURES_OK is set by the driver, I guess it means "the device
>>> has offered the feature";
>>
>> For me this part is ok since it clarify that it's the driver that set
>> the bit.
>>
>>
>>
>>> during normal usage, it means "the feature
>>> has been negotiated".
>> /?
>>
>> It looks to me the feature negotiation is done only after device set
>> FEATURES_OK, or FEATURES_OK could be read from device status?
> I'd consider feature negotiation done when the driver reads FEATURES_OK
> back from the status.


I agree.


>
>>
>>>    (This is a bit fuzzy for legacy mode.)
> ...because legacy does not have FEATURES_OK.
>    
>>
>> The problem is the MTU description for example:
>>
>> "The following driver-read-only field, mtu only exists if
>> VIRTIO_NET_F_MTU is set."
>>
>> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".
> "offered by the device"? I don't think it should 'disappear' from the
> config space if the driver won't use it. (Same for other config space
> fields that are tied to feature bits.)


But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks 
to according to the spec there will be no mtu field.

And a more interesting case is VIRTIO_NET_F_MQ is not offered but 
VIRTIO_NET_F_MTU offered. To me, it means we don't have 
max_virtqueue_pairs but it's not how the driver is wrote today.


>   
>> Otherwise readers (at least for me), may think the MTU is only valid
>> if driver set the bit.
> I think it would still be 'valid' in the sense that it exists and has
> some value in there filled in by the device, but a driver reading it
> without negotiating the feature would be buggy. (Like in the kernel
> code above; the kernel not liking the value does not make the field
> invalid.)


See Michael's reply, the spec allows read the config before setting 
features.


>
> Maybe a statement covering everything would be:
>
> "The following driver-read-only field mtu only exists if the device
> offers VIRTIO_NET_F_MTU and may be read by the driver during feature
> negotiation and after VIRTIO_NET_F_MTU has been successfully
> negotiated."
>
>>
>>> Should we add a wording clarification to the spec?
>>
>> I think so.
> Some clarification would be needed for each field that depends on a
> feature; that would be quite verbose. Maybe we can get away with a
> clarifying statement?
>
> "Some config space fields may depend on a certain feature. In that
> case, the field exits if the device has offered the corresponding
> feature,


So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config 
will look like:

struct virtio_net_config {
         u8 mac[6];
         le16 status;
         le16 mtu;
};


>   and may be read by the driver during feature negotiation, and
> accessed by the driver after the feature has been successfully
> negotiated. A shorthand for this is a statement that a field only
> exists if a certain feature bit is set."


I'm not sure using "shorthand" is good for the spec, at least we can 
limit the its scope only to the configuration space part.

Thanks


>

Jason Wang Feb. 24, 2021, 9:30 a.m. UTC | #38

On 2021/2/24 4:43 下午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 04:26:43PM +0800, Jason Wang wrote:
>>      Basically on first guest access QEMU would tell kernel whether
>>      guest is using the legacy or the modern interface.
>>      E.g. virtio_pci_config_read/virtio_pci_config_write will call ioctl(ENABLE_LEGACY, 1)
>>      while virtio_pci_common_read will call ioctl(ENABLE_LEGACY, 0)
>>
>>
>> But this trick work only for PCI I think?
>>
>> Thanks
> ccw has a revision it can check. mmio does not have transitional devices
> at all.


Ok, then we can do the workaround in the qemu, isn't it?

Thanks

Jason Wang Feb. 24, 2021, 9:40 a.m. UTC | #39

On 2021/2/23 6:17 下午, Jason Wang wrote:
>
> On 2021/2/23 6:01 下午, Michael S. Tsirkin wrote:
>> On Tue, Feb 23, 2021 at 05:46:20PM +0800, Jason Wang wrote:
>>> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
>>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
>>>>> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>> features to 0, when config space is accessed before features
>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>> and allow features reset to 0 for this case.
>>>>>>>
>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>> config space before features are set. For instance, when
>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>> will try to access and validate the MTU present in the config
>>>>>>> space before virtio features are set.
>>>>>> This looks like a spec violation:
>>>>>>
>>>>>> "
>>>>>>
>>>>>> The following driver-read-only field, mtu only exists if
>>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for 
>>>>>> the
>>>>>> driver to use.
>>>>>> "
>>>>>>
>>>>>> Do we really want to workaround this?
>>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy 
>>>>> guest?
>>>>>
>>>>> I think the point is, since there's legacy guest we'd have to 
>>>>> support, this
>>>>> host side workaround is unavoidable. Although I agree the 
>>>>> violating driver
>>>>> should be fixed (yes, it's in today's upstream kernel which exists 
>>>>> for a
>>>>> while now).
>>>> Oh  you are right:
>>>>
>>>>
>>>> static int virtnet_validate(struct virtio_device *vdev)
>>>> {
>>>>           if (!vdev->config->get) {
>>>>                   dev_err(&vdev->dev, "%s failure: config access 
>>>> disabled\n",
>>>>                           __func__);
>>>>                   return -EINVAL;
>>>>           }
>>>>
>>>>           if (!virtnet_validate_features(vdev))
>>>>                   return -EINVAL;
>>>>
>>>>           if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>>>                   int mtu = virtio_cread16(vdev,
>>>>                                            offsetof(struct 
>>>> virtio_net_config,
>>>>                                                     mtu));
>>>>                   if (mtu < MIN_MTU)
>>>>                           __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>>
>>> I wonder why not simply fail here?
>> Back in 2016 it went like this:
>>
>>     On Thu, Jun 02, 2016 at 05:10:59PM -0400, Aaron Conole wrote:
>>     > +     if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>     > +             dev->mtu = virtio_cread16(vdev,
>>     > +                                       offsetof(struct 
>> virtio_net_config,
>>     > +                                                mtu));
>>     > +     }
>>     > +
>>     >       if (vi->any_header_sg)
>>     >               dev->needed_headroom = vi->hdr_len;
>>     >
>>
>>     One comment though: I think we should validate the mtu.
>>     If it's invalid, clear VIRTIO_NET_F_MTU and ignore.
>>
>>
>> Too late at this point :)
>>
>> I guess it's a way to tell device "I can not live with this MTU",
>> device can fail FEATURES_OK if it wants to. MIN_MTU
>> is an internal linux thing and at the time I felt it's better to
>> try to make progress.
>
>
> What if e.g the device advertise a large MTU. E.g 64K here?


Ok, consider we use add_recvbuf_small() when neither GSO nor mrg_rxbuf. 
This means we should fail the probing if MTU is greater than 1500 in 
this case.

Thanks


> In that case, the driver can not live either. Clearing MTU won't help 
> here.
>
> Thanks
>
>
>>
>>
>>>>           }
>>>>
>>>>           return 0;
>>>> }
>>>>
>>>> And the spec says:
>>>>
>>>>
>>>> The driver MUST follow this sequence to initialize a device:
>>>> 1. Reset the device.
>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the 
>>>> device.
>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the 
>>>> device.
>>>> 4. Read device feature bits, and write the subset of feature bits 
>>>> understood by the OS and driver to the
>>>> device. During this step the driver MAY read (but MUST NOT write) 
>>>> the device-specific configuration
>>>> fields to check that it can support the device before accepting it.
>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new 
>>>> feature bits after this step.
>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still 
>>>> set: otherwise, the device does not
>>>> support our subset of features and the device is unusable.
>>>> 7. Perform device-specific setup, including discovery of virtqueues 
>>>> for the device, optional per-bus setup,
>>>> reading and possibly writing the device’s virtio configuration 
>>>> space, and population of virtqueues.
>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>
>>>>
>>>> Item 4 on the list explicitly allows reading config space before
>>>> FEATURES_OK.
>>>>
>>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device 
>>>> features".
>>>
>>> So this probably need some clarification. "is set" is used many 
>>> times in the
>>> spec that has different implications.
>>>
>>> Thanks
>>>
>>>
>>>> Generally it is worth going over feature dependent config fields
>>>> and checking whether they should be present when device feature is set
>>>> or when feature bit has been negotiated, and making this clear.
>>>>
>

Cornelia Huck Feb. 24, 2021, 11:12 a.m. UTC | #40

On Wed, 24 Feb 2021 17:29:07 +0800
Jason Wang <[e-mail]> wrote:

> On 2021/2/23 6:58 下午, Cornelia Huck wrote:
> > On Tue, 23 Feb 2021 18:31:07 +0800
> > Jason Wang <[e-mail]> wrote:
> >  
> >> On 2021/2/23 6:04 下午, Cornelia Huck wrote:  
> >>> On Tue, 23 Feb 2021 17:46:20 +0800
> >>> Jason Wang <[e-mail]> wrote:
> >>>     
> >>>> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:  
> >>>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:  
> >>>>>> On 2/21/2021 8:14 PM, Jason Wang wrote:  
> >>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:  
> >>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> >>>>>>>> for legacy") made an exception for legacy guests to reset
> >>>>>>>> features to 0, when config space is accessed before features
> >>>>>>>> are set. We should relieve the verify_min_features() check
> >>>>>>>> and allow features reset to 0 for this case.
> >>>>>>>>
> >>>>>>>> It's worth noting that not just legacy guests could access
> >>>>>>>> config space before features are set. For instance, when
> >>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
> >>>>>>>> will try to access and validate the MTU present in the config
> >>>>>>>> space before virtio features are set.  
> >>>>>>> This looks like a spec violation:
> >>>>>>>
> >>>>>>> "
> >>>>>>>
> >>>>>>> The following driver-read-only field, mtu only exists if
> >>>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
> >>>>>>> driver to use.
> >>>>>>> "
> >>>>>>>
> >>>>>>> Do we really want to workaround this?  
> >>>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
> >>>>>>
> >>>>>> I think the point is, since there's legacy guest we'd have to support, this
> >>>>>> host side workaround is unavoidable. Although I agree the violating driver
> >>>>>> should be fixed (yes, it's in today's upstream kernel which exists for a
> >>>>>> while now).  
> >>>>> Oh  you are right:
> >>>>>
> >>>>>
> >>>>> static int virtnet_validate(struct virtio_device *vdev)
> >>>>> {
> >>>>>            if (!vdev->config->get) {
> >>>>>                    dev_err(&vdev->dev, "%s failure: config access disabled\n",
> >>>>>                            __func__);
> >>>>>                    return -EINVAL;
> >>>>>            }
> >>>>>
> >>>>>            if (!virtnet_validate_features(vdev))
> >>>>>                    return -EINVAL;
> >>>>>
> >>>>>            if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> >>>>>                    int mtu = virtio_cread16(vdev,
> >>>>>                                             offsetof(struct virtio_net_config,
> >>>>>                                                      mtu));
> >>>>>                    if (mtu < MIN_MTU)
> >>>>>                            __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);  
> >>>> I wonder why not simply fail here?  
> >>> I think both failing or not accepting the feature can be argued to make
> >>> sense: "the device presented us with a mtu size that does not make
> >>> sense" would point to failing, "we cannot work with the mtu size that
> >>> the device presented us" would point to not negotiating the feature.
> >>>     
> >>>>     
> >>>>>            }
> >>>>>
> >>>>>            return 0;
> >>>>> }
> >>>>>
> >>>>> And the spec says:
> >>>>>
> >>>>>
> >>>>> The driver MUST follow this sequence to initialize a device:
> >>>>> 1. Reset the device.
> >>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> >>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> >>>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> >>>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> >>>>> fields to check that it can support the device before accepting it.
> >>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> >>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> >>>>> support our subset of features and the device is unusable.
> >>>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> >>>>> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> >>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> >>>>>
> >>>>>
> >>>>> Item 4 on the list explicitly allows reading config space before
> >>>>> FEATURES_OK.
> >>>>>
> >>>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".  
> >>>> So this probably need some clarification. "is set" is used many times in
> >>>> the spec that has different implications.  
> >>> Before FEATURES_OK is set by the driver, I guess it means "the device
> >>> has offered the feature";  
> >>
> >> For me this part is ok since it clarify that it's the driver that set
> >> the bit.
> >>
> >>
> >>  
> >>> during normal usage, it means "the feature
> >>> has been negotiated".  
> >> /?
> >>
> >> It looks to me the feature negotiation is done only after device set
> >> FEATURES_OK, or FEATURES_OK could be read from device status?  
> > I'd consider feature negotiation done when the driver reads FEATURES_OK
> > back from the status.  
> 
> 
> I agree.
> 
> 
> >  
> >>  
> >>>    (This is a bit fuzzy for legacy mode.)  
> > ...because legacy does not have FEATURES_OK.
> >      
> >>
> >> The problem is the MTU description for example:
> >>
> >> "The following driver-read-only field, mtu only exists if
> >> VIRTIO_NET_F_MTU is set."
> >>
> >> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".  
> > "offered by the device"? I don't think it should 'disappear' from the
> > config space if the driver won't use it. (Same for other config space
> > fields that are tied to feature bits.)  
> 
> 
> But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks 
> to according to the spec there will be no mtu field.

I think so, yes.

> 
> And a more interesting case is VIRTIO_NET_F_MQ is not offered but 
> VIRTIO_NET_F_MTU offered. To me, it means we don't have 
> max_virtqueue_pairs but it's not how the driver is wrote today.

That would be a bug, but it seems to me that the virtio-net driver
reads max_virtqueue_pairs conditionally and handles absence of the
feature correctly?

> 
> 
> >     
> >> Otherwise readers (at least for me), may think the MTU is only valid
> >> if driver set the bit.  
> > I think it would still be 'valid' in the sense that it exists and has
> > some value in there filled in by the device, but a driver reading it
> > without negotiating the feature would be buggy. (Like in the kernel
> > code above; the kernel not liking the value does not make the field
> > invalid.)  
> 
> 
> See Michael's reply, the spec allows read the config before setting 
> features.

Yes, the period prior to finishing negotiation is obviously special.

> 
> 
> >
> > Maybe a statement covering everything would be:
> >
> > "The following driver-read-only field mtu only exists if the device
> > offers VIRTIO_NET_F_MTU and may be read by the driver during feature
> > negotiation and after VIRTIO_NET_F_MTU has been successfully
> > negotiated."
> >  
> >>  
> >>> Should we add a wording clarification to the spec?  
> >>
> >> I think so.  
> > Some clarification would be needed for each field that depends on a
> > feature; that would be quite verbose. Maybe we can get away with a
> > clarifying statement?
> >
> > "Some config space fields may depend on a certain feature. In that
> > case, the field exits if the device has offered the corresponding
> > feature,  
> 
> 
> So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config 
> will look like:
> 
> struct virtio_net_config {
>          u8 mac[6];
>          le16 status;
>          le16 mtu;
> };
> 

I agree.

> 
> >   and may be read by the driver during feature negotiation, and
> > accessed by the driver after the feature has been successfully
> > negotiated. A shorthand for this is a statement that a field only
> > exists if a certain feature bit is set."  
> 
> 
> I'm not sure using "shorthand" is good for the spec, at least we can 
> limit the its scope only to the configuration space part.

Maybe "a shorthand expression"?

Eli Cohen Feb. 24, 2021, 12:40 p.m. UTC | #41

On Wed, Feb 24, 2021 at 02:12:01AM -0500, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 02:55:13PM +0800, Jason Wang wrote:
> > 
> > On 2021/2/24 2:47 下午, Michael S. Tsirkin wrote:
> > > On Wed, Feb 24, 2021 at 08:45:20AM +0200, Eli Cohen wrote:
> > > > On Wed, Feb 24, 2021 at 12:17:58AM -0500, Michael S. Tsirkin wrote:
> > > > > On Wed, Feb 24, 2021 at 11:20:01AM +0800, Jason Wang wrote:
> > > > > > On 2021/2/24 3:35 上午, Si-Wei Liu wrote:
> > > > > > > 
> > > > > > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > > > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > > > > > space before virtio features are set.
> > > > > > > > > > > > This looks like a spec violation:
> > > > > > > > > > > > 
> > > > > > > > > > > > "
> > > > > > > > > > > > 
> > > > > > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > > > > > set.
> > > > > > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > > > > > "
> > > > > > > > > > > > 
> > > > > > > > > > > > Do we really want to workaround this?
> > > > > > > > > > > > 
> > > > > > > > > > > > Thanks
> > > > > > > > > > > And also:
> > > > > > > > > > > 
> > > > > > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > > > > > 1. Reset the device.
> > > > > > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has
> > > > > > > > > > > noticed the device.
> > > > > > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > > > > > device.
> > > > > > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > > > > > understood by the OS and driver to the
> > > > > > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > > > > > the device-specific configuration
> > > > > > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > > > > > feature bits after this step.
> > > > > > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > > > > > otherwise, the device does not
> > > > > > > > > > > support our subset of features and the device is unusable.
> > > > > > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > > > > > for the device, optional per-bus setup,
> > > > > > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > > > > > space, and population of virtqueues.
> > > > > > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > > so accessing config space before FEATURES_OK is a spec
> > > > > > > > > > > violation, right?
> > > > > > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > > > > > thought the legacy guest still needs to be supported.
> > > > > > > > > > 
> > > > > > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > > > > > issue where this discrepancy is introduced to
> > > > > > > > > > virtnet_validate() (since
> > > > > > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > > > > > 
> > > > > > > > > > -Siwei
> > > > > > > > > I think it's a bug to read config space in validate, we should
> > > > > > > > > move it to
> > > > > > > > > virtnet_probe().
> > > > > > > > > 
> > > > > > > > > Thanks
> > > > > > > > I take it back, reading but not writing seems to be explicitly
> > > > > > > > allowed by spec.
> > > > > > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > > > > > the best way to handle this.
> > > > > > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect
> > > > > > > legacy guest? Supposedly only config space write access needs to be
> > > > > > > guarded before setting FEATURES_OK.
> > > > > > 
> > > > > > I agree. My understanding is that all vDPA must be modern device (since
> > > > > > VIRITO_F_ACCESS_PLATFORM is mandated) instead of transitional device.
> > > > > > 
> > > > > > Thanks
> > > > > Well mlx5 has some code to handle legacy guests ...
> > > > > Eli, could you comment? Is that support unused right now?
> > > > > 
> > > > If you mean support for version 1.0, well the knob is there but it's not
> > > > set in the firmware I use. Note sure if we will support this.
> > > Hmm you mean it's legacy only right now?
> > > Well at some point you will want advanced goodies like RSS
> > > and all that is gated on 1.0 ;)
> > 

Guys sorry, I checked again, the firmware capability is set and so is
VIRTIO_F_VERSION_1.

> > 
> > So if my understanding is correct the device/firmware is legacy but require
> > VIRTIO_F_ACCESS_PLATFORM semanic? Looks like a spec violation?
> > 
> > Thanks
> 
> Legacy mode description is the spec is non-normative. As such as long as
> guests work, they work ;)
> 
> > 
> > > 
> > > > > > > -Siwie
> > > > > > > 
> > > > > > > > > > > > > Rejecting reset to 0
> > > > > > > > > > > > > prematurely causes correct MTU and link status unable to load
> > > > > > > > > > > > > for the very first config space access, rendering issues like
> > > > > > > > > > > > > guest showing inaccurate MTU value, or failure to reject
> > > > > > > > > > > > > out-of-range MTU.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
> > > > > > > > > > > > > supported mlx5 devices")
> > > > > > > > > > > > > Signed-off-by: Si-Wei Liu <[e-mail]>
> > > > > > > > > > > > > ---
> > > > > > > > > > > > >      drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
> > > > > > > > > > > > >      1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > > > > > > > > > 
> > > > > > > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > > index 7c1f789..540dd67 100644
> > > > > > > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > > @@ -1490,14 +1490,6 @@ static u64
> > > > > > > > > > > > > mlx5_vdpa_get_features(struct vdpa_device *vdev)
> > > > > > > > > > > > >          return mvdev->mlx_features;
> > > > > > > > > > > > >      }
> > > > > > > > > > > > > -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
> > > > > > > > > > > > > u64 features)
> > > > > > > > > > > > > -{
> > > > > > > > > > > > > -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
> > > > > > > > > > > > > -        return -EOPNOTSUPP;
> > > > > > > > > > > > > -
> > > > > > > > > > > > > -    return 0;
> > > > > > > > > > > > > -}
> > > > > > > > > > > > > -
> > > > > > > > > > > > >      static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
> > > > > > > > > > > > >      {
> > > > > > > > > > > > >          int err;
> > > > > > > > > > > > > @@ -1558,18 +1550,13 @@ static int
> > > > > > > > > > > > > mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
> > > > > > > > > > > > > features)
> > > > > > > > > > > > >      {
> > > > > > > > > > > > >          struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > > > > > > > > > >          struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
> > > > > > > > > > > > > -    int err;
> > > > > > > > > > > > >          print_features(mvdev, features, true);
> > > > > > > > > > > > > -    err = verify_min_features(mvdev, features);
> > > > > > > > > > > > > -    if (err)
> > > > > > > > > > > > > -        return err;
> > > > > > > > > > > > > -
> > > > > > > > > > > > >          ndev->mvdev.actual_features = features &
> > > > > > > > > > > > > ndev->mvdev.mlx_features;
> > > > > > > > > > > > >          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
> > > > > > > > > > > > >          ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
> > > > > > > > > > > > > VIRTIO_NET_S_LINK_UP);
> > > > > > > > > > > > > -    return err;
> > > > > > > > > > > > > +    return 0;
> > > > > > > > > > > > >      }
> > > > > > > > > > > > >      static void mlx5_vdpa_set_config_cb(struct vdpa_device
> > > > > > > > > > > > > *vdev, struct vdpa_callback *cb)
>

Si-Wei Liu Feb. 24, 2021, 6:24 p.m. UTC | #42

On 2/23/2021 9:04 PM, Michael S. Tsirkin wrote:
> On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
>>
>> On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
>>> On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
>>>> On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
>>>>> On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
>>>>>> On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>
>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>> config space before features are set. For instance, when
>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>> space before virtio features are set.
>>>>>>> This looks like a spec violation:
>>>>>>>
>>>>>>> "
>>>>>>>
>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>> VIRTIO_NET_F_MTU is
>>>>>>> set.
>>>>>>> This field specifies the maximum MTU for the driver to use.
>>>>>>> "
>>>>>>>
>>>>>>> Do we really want to workaround this?
>>>>>>>
>>>>>>> Thanks
>>>>>> And also:
>>>>>>
>>>>>> The driver MUST follow this sequence to initialize a device:
>>>>>> 1. Reset the device.
>>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the
>>>>>> device.
>>>>>> 4. Read device feature bits, and write the subset of feature bits
>>>>>> understood by the OS and driver to the
>>>>>> device. During this step the driver MAY read (but MUST NOT write)
>>>>>> the device-specific configuration
>>>>>> fields to check that it can support the device before accepting it.
>>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
>>>>>> feature bits after this step.
>>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set:
>>>>>> otherwise, the device does not
>>>>>> support our subset of features and the device is unusable.
>>>>>> 7. Perform device-specific setup, including discovery of virtqueues
>>>>>> for the device, optional per-bus setup,
>>>>>> reading and possibly writing the device’s virtio configuration
>>>>>> space, and population of virtqueues.
>>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>>
>>>>>>
>>>>>> so accessing config space before FEATURES_OK is a spec violation, right?
>>>>> It is, but it's not relevant to what this commit tries to address. I
>>>>> thought the legacy guest still needs to be supported.
>>>>>
>>>>> Having said, a separate patch has to be posted to fix the guest driver
>>>>> issue where this discrepancy is introduced to virtnet_validate() (since
>>>>> commit fe36cbe067). But it's not technically related to this patch.
>>>>>
>>>>> -Siwei
>>>> I think it's a bug to read config space in validate, we should move it to
>>>> virtnet_probe().
>>>>
>>>> Thanks
>>> I take it back, reading but not writing seems to be explicitly allowed by spec.
>>> So our way to detect a legacy guest is bogus, need to think what is
>>> the best way to handle this.
>> Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
>> guest? Supposedly only config space write access needs to be guarded before
>> setting FEATURES_OK.
>>
>> -Siwie
> Detecting it isn't enough though, we will need a new ioctl to notify
> the kernel that it's a legacy guest. Ugh :(
Well, although I think adding an ioctl is doable, may I know what the 
use case there will be for kernel to leverage such info directly? Is 
there a case QEMU can't do with dedicate ioctls later if there's indeed 
differentiation (legacy v.s. modern) needed?

One of the reason I asked is if this ioctl becomes a mandate for 
vhost-vdpa kernel. QEMU would reject initialize vhost-vdpa if doesn't 
see this ioctl coming?

If it's optional, suppose the kernel may need it only when it becomes 
necessary?

Thanks,
-Siwei


>
>
>>>>>>>> Rejecting reset to 0
>>>>>>>> prematurely causes correct MTU and link status unable to load
>>>>>>>> for the very first config space access, rendering issues like
>>>>>>>> guest showing inaccurate MTU value, or failure to reject
>>>>>>>> out-of-range MTU.
>>>>>>>>
>>>>>>>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for
>>>>>>>> supported mlx5 devices")
>>>>>>>> Signed-off-by: Si-Wei Liu <[e-mail]>
>>>>>>>> ---
>>>>>>>>      drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>>>>>>>      1 file changed, 1 insertion(+), 14 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>> index 7c1f789..540dd67 100644
>>>>>>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>>>>>>> @@ -1490,14 +1490,6 @@ static u64
>>>>>>>> mlx5_vdpa_get_features(struct vdpa_device *vdev)
>>>>>>>>          return mvdev->mlx_features;
>>>>>>>>      }
>>>>>>>> -static int verify_min_features(struct mlx5_vdpa_dev *mvdev,
>>>>>>>> u64 features)
>>>>>>>> -{
>>>>>>>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>>>>>>>> -        return -EOPNOTSUPP;
>>>>>>>> -
>>>>>>>> -    return 0;
>>>>>>>> -}
>>>>>>>> -
>>>>>>>>      static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>>>>>>>      {
>>>>>>>>          int err;
>>>>>>>> @@ -1558,18 +1550,13 @@ static int
>>>>>>>> mlx5_vdpa_set_features(struct vdpa_device *vdev, u64
>>>>>>>> features)
>>>>>>>>      {
>>>>>>>>          struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>>>>>>          struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>>>>>>> -    int err;
>>>>>>>>          print_features(mvdev, features, true);
>>>>>>>> -    err = verify_min_features(mvdev, features);
>>>>>>>> -    if (err)
>>>>>>>> -        return err;
>>>>>>>> -
>>>>>>>>          ndev->mvdev.actual_features = features &
>>>>>>>> ndev->mvdev.mlx_features;
>>>>>>>>          ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>>>>>>>          ndev->config.status |= cpu_to_mlx5vdpa16(mvdev,
>>>>>>>> VIRTIO_NET_S_LINK_UP);
>>>>>>>> -    return err;
>>>>>>>> +    return 0;
>>>>>>>>      }
>>>>>>>>      static void mlx5_vdpa_set_config_cb(struct vdpa_device
>>>>>>>> *vdev, struct vdpa_callback *cb)

Jason Wang Feb. 25, 2021, 4:36 a.m. UTC | #43

On 2021/2/24 7:12 下午, Cornelia Huck wrote:
> On Wed, 24 Feb 2021 17:29:07 +0800
> Jason Wang <[e-mail]> wrote:
>
>> On 2021/2/23 6:58 下午, Cornelia Huck wrote:
>>> On Tue, 23 Feb 2021 18:31:07 +0800
>>> Jason Wang <[e-mail]> wrote:
>>>   
>>>> On 2021/2/23 6:04 下午, Cornelia Huck wrote:
>>>>> On Tue, 23 Feb 2021 17:46:20 +0800
>>>>> Jason Wang <[e-mail]> wrote:
>>>>>      
>>>>>> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
>>>>>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
>>>>>>>> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>>>
>>>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>>>> config space before features are set. For instance, when
>>>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>>>> space before virtio features are set.
>>>>>>>>> This looks like a spec violation:
>>>>>>>>>
>>>>>>>>> "
>>>>>>>>>
>>>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
>>>>>>>>> driver to use.
>>>>>>>>> "
>>>>>>>>>
>>>>>>>>> Do we really want to workaround this?
>>>>>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
>>>>>>>>
>>>>>>>> I think the point is, since there's legacy guest we'd have to support, this
>>>>>>>> host side workaround is unavoidable. Although I agree the violating driver
>>>>>>>> should be fixed (yes, it's in today's upstream kernel which exists for a
>>>>>>>> while now).
>>>>>>> Oh  you are right:
>>>>>>>
>>>>>>>
>>>>>>> static int virtnet_validate(struct virtio_device *vdev)
>>>>>>> {
>>>>>>>             if (!vdev->config->get) {
>>>>>>>                     dev_err(&vdev->dev, "%s failure: config access disabled\n",
>>>>>>>                             __func__);
>>>>>>>                     return -EINVAL;
>>>>>>>             }
>>>>>>>
>>>>>>>             if (!virtnet_validate_features(vdev))
>>>>>>>                     return -EINVAL;
>>>>>>>
>>>>>>>             if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>>>>>>                     int mtu = virtio_cread16(vdev,
>>>>>>>                                              offsetof(struct virtio_net_config,
>>>>>>>                                                       mtu));
>>>>>>>                     if (mtu < MIN_MTU)
>>>>>>>                             __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>>>>> I wonder why not simply fail here?
>>>>> I think both failing or not accepting the feature can be argued to make
>>>>> sense: "the device presented us with a mtu size that does not make
>>>>> sense" would point to failing, "we cannot work with the mtu size that
>>>>> the device presented us" would point to not negotiating the feature.
>>>>>      
>>>>>>      
>>>>>>>             }
>>>>>>>
>>>>>>>             return 0;
>>>>>>> }
>>>>>>>
>>>>>>> And the spec says:
>>>>>>>
>>>>>>>
>>>>>>> The driver MUST follow this sequence to initialize a device:
>>>>>>> 1. Reset the device.
>>>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
>>>>>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
>>>>>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
>>>>>>> fields to check that it can support the device before accepting it.
>>>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
>>>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
>>>>>>> support our subset of features and the device is unusable.
>>>>>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
>>>>>>> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
>>>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>>>
>>>>>>>
>>>>>>> Item 4 on the list explicitly allows reading config space before
>>>>>>> FEATURES_OK.
>>>>>>>
>>>>>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
>>>>>> So this probably need some clarification. "is set" is used many times in
>>>>>> the spec that has different implications.
>>>>> Before FEATURES_OK is set by the driver, I guess it means "the device
>>>>> has offered the feature";
>>>> For me this part is ok since it clarify that it's the driver that set
>>>> the bit.
>>>>
>>>>
>>>>   
>>>>> during normal usage, it means "the feature
>>>>> has been negotiated".
>>>> /?
>>>>
>>>> It looks to me the feature negotiation is done only after device set
>>>> FEATURES_OK, or FEATURES_OK could be read from device status?
>>> I'd consider feature negotiation done when the driver reads FEATURES_OK
>>> back from the status.
>>
>> I agree.
>>
>>
>>>   
>>>>   
>>>>>     (This is a bit fuzzy for legacy mode.)
>>> ...because legacy does not have FEATURES_OK.
>>>       
>>>> The problem is the MTU description for example:
>>>>
>>>> "The following driver-read-only field, mtu only exists if
>>>> VIRTIO_NET_F_MTU is set."
>>>>
>>>> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".
>>> "offered by the device"? I don't think it should 'disappear' from the
>>> config space if the driver won't use it. (Same for other config space
>>> fields that are tied to feature bits.)
>>
>> But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks
>> to according to the spec there will be no mtu field.
> I think so, yes.
>
>> And a more interesting case is VIRTIO_NET_F_MQ is not offered but
>> VIRTIO_NET_F_MTU offered. To me, it means we don't have
>> max_virtqueue_pairs but it's not how the driver is wrote today.
> That would be a bug, but it seems to me that the virtio-net driver
> reads max_virtqueue_pairs conditionally and handles absence of the
> feature correctly?


Yes, see the avove codes:

         if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
                 int mtu = virtio_cread16(vdev,
                                          offsetof(struct virtio_net_config,
                                                   mtu));
                 if (mtu < MIN_MTU)
                         __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
         }

So it's probably too late to fix the driver.


>
>>
>>>      
>>>> Otherwise readers (at least for me), may think the MTU is only valid
>>>> if driver set the bit.
>>> I think it would still be 'valid' in the sense that it exists and has
>>> some value in there filled in by the device, but a driver reading it
>>> without negotiating the feature would be buggy. (Like in the kernel
>>> code above; the kernel not liking the value does not make the field
>>> invalid.)
>>
>> See Michael's reply, the spec allows read the config before setting
>> features.
> Yes, the period prior to finishing negotiation is obviously special.
>
>>
>>> Maybe a statement covering everything would be:
>>>
>>> "The following driver-read-only field mtu only exists if the device
>>> offers VIRTIO_NET_F_MTU and may be read by the driver during feature
>>> negotiation and after VIRTIO_NET_F_MTU has been successfully
>>> negotiated."
>>>   
>>>>   
>>>>> Should we add a wording clarification to the spec?
>>>> I think so.
>>> Some clarification would be needed for each field that depends on a
>>> feature; that would be quite verbose. Maybe we can get away with a
>>> clarifying statement?
>>>
>>> "Some config space fields may depend on a certain feature. In that
>>> case, the field exits if the device has offered the corresponding
>>> feature,
>>
>> So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config
>> will look like:
>>
>> struct virtio_net_config {
>>           u8 mac[6];
>>           le16 status;
>>           le16 mtu;
>> };
>>
> I agree.


So consider it's probably too late to fix the driver which assumes some 
field are always persent, it looks to me need fix the spec do declare 
the fields are always existing instead.


>
>>>    and may be read by the driver during feature negotiation, and
>>> accessed by the driver after the feature has been successfully
>>> negotiated. A shorthand for this is a statement that a field only
>>> exists if a certain feature bit is set."
>>
>> I'm not sure using "shorthand" is good for the spec, at least we can
>> limit the its scope only to the configuration space part.
> Maybe "a shorthand expression"?


So the questions is should we use this for all over the spec or it will 
be only used in this speicifc part (device configuration).

Thanks

Cornelia Huck Feb. 25, 2021, 1:26 p.m. UTC | #44

On Thu, 25 Feb 2021 12:36:07 +0800
Jason Wang <[e-mail]> wrote:

> On 2021/2/24 7:12 下午, Cornelia Huck wrote:
> > On Wed, 24 Feb 2021 17:29:07 +0800
> > Jason Wang <[e-mail]> wrote:
> >  
> >> On 2021/2/23 6:58 下午, Cornelia Huck wrote:  
> >>> On Tue, 23 Feb 2021 18:31:07 +0800
> >>> Jason Wang <[e-mail]> wrote:

> >>>> The problem is the MTU description for example:
> >>>>
> >>>> "The following driver-read-only field, mtu only exists if
> >>>> VIRTIO_NET_F_MTU is set."
> >>>>
> >>>> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".  
> >>> "offered by the device"? I don't think it should 'disappear' from the
> >>> config space if the driver won't use it. (Same for other config space
> >>> fields that are tied to feature bits.)  
> >>
> >> But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks
> >> to according to the spec there will be no mtu field.  
> > I think so, yes.
> >  
> >> And a more interesting case is VIRTIO_NET_F_MQ is not offered but
> >> VIRTIO_NET_F_MTU offered. To me, it means we don't have
> >> max_virtqueue_pairs but it's not how the driver is wrote today.  
> > That would be a bug, but it seems to me that the virtio-net driver
> > reads max_virtqueue_pairs conditionally and handles absence of the
> > feature correctly?  
> 
> 
> Yes, see the avove codes:
> 
>          if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>                  int mtu = virtio_cread16(vdev,
>                                           offsetof(struct virtio_net_config,
>                                                    mtu));
>                  if (mtu < MIN_MTU)
>                          __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>          }
> 

Ouch, you're right. The virtio_cread accessors all operate on offsets
into a structure, it's just more obvious here.

> So it's probably too late to fix the driver.

It is never too late to fix a driver :)

It seems involved, though. We'd need different config space structures
based upon which set of features has been negotiated. It's not too bad
when features build upon each other and add fields at the end (this
should be fine right now, if the code remembered to check for the
feature), but can become ugly if an in-between field depends upon a
feature.

I guess we've been lucky that it seems to be an extremely uncommon
configuration.

> 
> 
> >  
> >>  
> >>>        
> >>>> Otherwise readers (at least for me), may think the MTU is only valid
> >>>> if driver set the bit.  
> >>> I think it would still be 'valid' in the sense that it exists and has
> >>> some value in there filled in by the device, but a driver reading it
> >>> without negotiating the feature would be buggy. (Like in the kernel
> >>> code above; the kernel not liking the value does not make the field
> >>> invalid.)  
> >>
> >> See Michael's reply, the spec allows read the config before setting
> >> features.  
> > Yes, the period prior to finishing negotiation is obviously special.
> >  
> >>  
> >>> Maybe a statement covering everything would be:
> >>>
> >>> "The following driver-read-only field mtu only exists if the device
> >>> offers VIRTIO_NET_F_MTU and may be read by the driver during feature
> >>> negotiation and after VIRTIO_NET_F_MTU has been successfully
> >>> negotiated."
> >>>     
> >>>>     
> >>>>> Should we add a wording clarification to the spec?  
> >>>> I think so.  
> >>> Some clarification would be needed for each field that depends on a
> >>> feature; that would be quite verbose. Maybe we can get away with a
> >>> clarifying statement?
> >>>
> >>> "Some config space fields may depend on a certain feature. In that
> >>> case, the field exits if the device has offered the corresponding
> >>> feature,  
> >>
> >> So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config
> >> will look like:
> >>
> >> struct virtio_net_config {
> >>           u8 mac[6];
> >>           le16 status;
> >>           le16 mtu;
> >> };
> >>  
> > I agree.  
> 
> 
> So consider it's probably too late to fix the driver which assumes some 
> field are always persent, it looks to me need fix the spec do declare 
> the fields are always existing instead.

The problem with that is that it has been in the spec already, and a
compliant device that did not provide the fields without the features
would suddenly become non-compliant...

> 
> 
> >  
> >>>    and may be read by the driver during feature negotiation, and
> >>> accessed by the driver after the feature has been successfully
> >>> negotiated. A shorthand for this is a statement that a field only
> >>> exists if a certain feature bit is set."  
> >>
> >> I'm not sure using "shorthand" is good for the spec, at least we can
> >> limit the its scope only to the configuration space part.  
> > Maybe "a shorthand expression"?  
> 
> 
> So the questions is should we use this for all over the spec or it will 
> be only used in this speicifc part (device configuration).

For command structures and the like, "feature is set" should always
mean "feature has been negotiated"; I think config space is special
because the driver can read it before feature negotiation is finished,
so device configuration is probably the proper place for it.

Michael S. Tsirkin Feb. 25, 2021, 6:53 p.m. UTC | #45

On Thu, Feb 25, 2021 at 12:36:07PM +0800, Jason Wang wrote:
> 
> On 2021/2/24 7:12 下午, Cornelia Huck wrote:
> > On Wed, 24 Feb 2021 17:29:07 +0800
> > Jason Wang <[e-mail]> wrote:
> > 
> > > On 2021/2/23 6:58 下午, Cornelia Huck wrote:
> > > > On Tue, 23 Feb 2021 18:31:07 +0800
> > > > Jason Wang <[e-mail]> wrote:
> > > > > On 2021/2/23 6:04 下午, Cornelia Huck wrote:
> > > > > > On Tue, 23 Feb 2021 17:46:20 +0800
> > > > > > Jason Wang <[e-mail]> wrote:
> > > > > > > On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
> > > > > > > > On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
> > > > > > > > > On 2/21/2021 8:14 PM, Jason Wang wrote:
> > > > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > > > 
> > > > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > > > space before virtio features are set.
> > > > > > > > > > This looks like a spec violation:
> > > > > > > > > > 
> > > > > > > > > > "
> > > > > > > > > > 
> > > > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > > > VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
> > > > > > > > > > driver to use.
> > > > > > > > > > "
> > > > > > > > > > 
> > > > > > > > > > Do we really want to workaround this?
> > > > > > > > > Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
> > > > > > > > > 
> > > > > > > > > I think the point is, since there's legacy guest we'd have to support, this
> > > > > > > > > host side workaround is unavoidable. Although I agree the violating driver
> > > > > > > > > should be fixed (yes, it's in today's upstream kernel which exists for a
> > > > > > > > > while now).
> > > > > > > > Oh  you are right:
> > > > > > > > 
> > > > > > > > 
> > > > > > > > static int virtnet_validate(struct virtio_device *vdev)
> > > > > > > > {
> > > > > > > >             if (!vdev->config->get) {
> > > > > > > >                     dev_err(&vdev->dev, "%s failure: config access disabled\n",
> > > > > > > >                             __func__);
> > > > > > > >                     return -EINVAL;
> > > > > > > >             }
> > > > > > > > 
> > > > > > > >             if (!virtnet_validate_features(vdev))
> > > > > > > >                     return -EINVAL;
> > > > > > > > 
> > > > > > > >             if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> > > > > > > >                     int mtu = virtio_cread16(vdev,
> > > > > > > >                                              offsetof(struct virtio_net_config,
> > > > > > > >                                                       mtu));
> > > > > > > >                     if (mtu < MIN_MTU)
> > > > > > > >                             __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
> > > > > > > I wonder why not simply fail here?
> > > > > > I think both failing or not accepting the feature can be argued to make
> > > > > > sense: "the device presented us with a mtu size that does not make
> > > > > > sense" would point to failing, "we cannot work with the mtu size that
> > > > > > the device presented us" would point to not negotiating the feature.
> > > > > > > >             }
> > > > > > > > 
> > > > > > > >             return 0;
> > > > > > > > }
> > > > > > > > 
> > > > > > > > And the spec says:
> > > > > > > > 
> > > > > > > > 
> > > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > > 1. Reset the device.
> > > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> > > > > > > > 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> > > > > > > > device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> > > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> > > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> > > > > > > > support our subset of features and the device is unusable.
> > > > > > > > 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> > > > > > > > reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> > > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > > 
> > > > > > > > 
> > > > > > > > Item 4 on the list explicitly allows reading config space before
> > > > > > > > FEATURES_OK.
> > > > > > > > 
> > > > > > > > I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
> > > > > > > So this probably need some clarification. "is set" is used many times in
> > > > > > > the spec that has different implications.
> > > > > > Before FEATURES_OK is set by the driver, I guess it means "the device
> > > > > > has offered the feature";
> > > > > For me this part is ok since it clarify that it's the driver that set
> > > > > the bit.
> > > > > 
> > > > > 
> > > > > > during normal usage, it means "the feature
> > > > > > has been negotiated".
> > > > > /?
> > > > > 
> > > > > It looks to me the feature negotiation is done only after device set
> > > > > FEATURES_OK, or FEATURES_OK could be read from device status?
> > > > I'd consider feature negotiation done when the driver reads FEATURES_OK
> > > > back from the status.
> > > 
> > > I agree.
> > > 
> > > 
> > > > > >     (This is a bit fuzzy for legacy mode.)
> > > > ...because legacy does not have FEATURES_OK.
> > > > > The problem is the MTU description for example:
> > > > > 
> > > > > "The following driver-read-only field, mtu only exists if
> > > > > VIRTIO_NET_F_MTU is set."
> > > > > 
> > > > > It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".
> > > > "offered by the device"? I don't think it should 'disappear' from the
> > > > config space if the driver won't use it. (Same for other config space
> > > > fields that are tied to feature bits.)
> > > 
> > > But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks
> > > to according to the spec there will be no mtu field.
> > I think so, yes.
> > 
> > > And a more interesting case is VIRTIO_NET_F_MQ is not offered but
> > > VIRTIO_NET_F_MTU offered. To me, it means we don't have
> > > max_virtqueue_pairs but it's not how the driver is wrote today.
> > That would be a bug, but it seems to me that the virtio-net driver
> > reads max_virtqueue_pairs conditionally and handles absence of the
> > feature correctly?
> 
> 
> Yes, see the avove codes:
> 
>         if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>                 int mtu = virtio_cread16(vdev,
>                                          offsetof(struct virtio_net_config,
>                                                   mtu));
>                 if (mtu < MIN_MTU)
>                         __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>         }
> 
> So it's probably too late to fix the driver.
> 

Confused. What is wrong with the above? It never reads the
field unless the feature has been offered by device.


> > 
> > > 
> > > > > Otherwise readers (at least for me), may think the MTU is only valid
> > > > > if driver set the bit.
> > > > I think it would still be 'valid' in the sense that it exists and has
> > > > some value in there filled in by the device, but a driver reading it
> > > > without negotiating the feature would be buggy. (Like in the kernel
> > > > code above; the kernel not liking the value does not make the field
> > > > invalid.)
> > > 
> > > See Michael's reply, the spec allows read the config before setting
> > > features.
> > Yes, the period prior to finishing negotiation is obviously special.
> > 
> > > 
> > > > Maybe a statement covering everything would be:
> > > > 
> > > > "The following driver-read-only field mtu only exists if the device
> > > > offers VIRTIO_NET_F_MTU and may be read by the driver during feature
> > > > negotiation and after VIRTIO_NET_F_MTU has been successfully
> > > > negotiated."
> > > > > > Should we add a wording clarification to the spec?
> > > > > I think so.
> > > > Some clarification would be needed for each field that depends on a
> > > > feature; that would be quite verbose. Maybe we can get away with a
> > > > clarifying statement?
> > > > 
> > > > "Some config space fields may depend on a certain feature. In that
> > > > case, the field exits if the device has offered the corresponding
> > > > feature,
> > > 
> > > So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config
> > > will look like:
> > > 
> > > struct virtio_net_config {
> > >           u8 mac[6];
> > >           le16 status;
> > >           le16 mtu;
> > > };
> > > 
> > I agree.
> 
> 
> So consider it's probably too late to fix the driver which assumes some
> field are always persent, it looks to me need fix the spec do declare the
> fields are always existing instead.
> 
> 
> > 
> > > >    and may be read by the driver during feature negotiation, and
> > > > accessed by the driver after the feature has been successfully
> > > > negotiated. A shorthand for this is a statement that a field only
> > > > exists if a certain feature bit is set."
> > > 
> > > I'm not sure using "shorthand" is good for the spec, at least we can
> > > limit the its scope only to the configuration space part.
> > Maybe "a shorthand expression"?
> 
> 
> So the questions is should we use this for all over the spec or it will be
> only used in this speicifc part (device configuration).
> 
> Thanks
>

Si-Wei Liu Feb. 26, 2021, 12:56 a.m. UTC | #46

Hi Michael,

Are you okay to live without this ioctl for now? I think QEMU is the one 
that needs to be fixed and will have to be made legacy guest aware. I 
think the kernel can just honor the feature negotiation result done by 
QEMU and do as what's told to.Will you agree?

If it's fine, I would proceed to reverting commit fe36cbe067 and related 
code in question from the kernel.

Thanks,
-Siwei

On 2/24/2021 10:24 AM, Si-Wei Liu wrote:
>> Detecting it isn't enough though, we will need a new ioctl to notify
>> the kernel that it's a legacy guest. Ugh :(
> Well, although I think adding an ioctl is doable, may I know what the 
> use case there will be for kernel to leverage such info directly? Is 
> there a case QEMU can't do with dedicate ioctls later if there's 
> indeed differentiation (legacy v.s. modern) needed?
>
> One of the reason I asked is if this ioctl becomes a mandate for 
> vhost-vdpa kernel. QEMU would reject initialize vhost-vdpa if doesn't 
> see this ioctl coming?
>
> If it's optional, suppose the kernel may need it only when it becomes 
> necessary?
>
> Thanks,
> -Siwei

Jason Wang Feb. 26, 2021, 8:19 a.m. UTC | #47

On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:
> On Thu, Feb 25, 2021 at 12:36:07PM +0800, Jason Wang wrote:
>> On 2021/2/24 7:12 下午, Cornelia Huck wrote:
>>> On Wed, 24 Feb 2021 17:29:07 +0800
>>> Jason Wang <[e-mail]> wrote:
>>>
>>>> On 2021/2/23 6:58 下午, Cornelia Huck wrote:
>>>>> On Tue, 23 Feb 2021 18:31:07 +0800
>>>>> Jason Wang <[e-mail]> wrote:
>>>>>> On 2021/2/23 6:04 下午, Cornelia Huck wrote:
>>>>>>> On Tue, 23 Feb 2021 17:46:20 +0800
>>>>>>> Jason Wang <[e-mail]> wrote:
>>>>>>>> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
>>>>>>>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
>>>>>>>>>> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>>>>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>>>>>
>>>>>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>>>>>> config space before features are set. For instance, when
>>>>>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>>>>>> space before virtio features are set.
>>>>>>>>>>> This looks like a spec violation:
>>>>>>>>>>>
>>>>>>>>>>> "
>>>>>>>>>>>
>>>>>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
>>>>>>>>>>> driver to use.
>>>>>>>>>>> "
>>>>>>>>>>>
>>>>>>>>>>> Do we really want to workaround this?
>>>>>>>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
>>>>>>>>>>
>>>>>>>>>> I think the point is, since there's legacy guest we'd have to support, this
>>>>>>>>>> host side workaround is unavoidable. Although I agree the violating driver
>>>>>>>>>> should be fixed (yes, it's in today's upstream kernel which exists for a
>>>>>>>>>> while now).
>>>>>>>>> Oh  you are right:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> static int virtnet_validate(struct virtio_device *vdev)
>>>>>>>>> {
>>>>>>>>>              if (!vdev->config->get) {
>>>>>>>>>                      dev_err(&vdev->dev, "%s failure: config access disabled\n",
>>>>>>>>>                              __func__);
>>>>>>>>>                      return -EINVAL;
>>>>>>>>>              }
>>>>>>>>>
>>>>>>>>>              if (!virtnet_validate_features(vdev))
>>>>>>>>>                      return -EINVAL;
>>>>>>>>>
>>>>>>>>>              if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>>>>>>>>                      int mtu = virtio_cread16(vdev,
>>>>>>>>>                                               offsetof(struct virtio_net_config,
>>>>>>>>>                                                        mtu));
>>>>>>>>>                      if (mtu < MIN_MTU)
>>>>>>>>>                              __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>>>>>>> I wonder why not simply fail here?
>>>>>>> I think both failing or not accepting the feature can be argued to make
>>>>>>> sense: "the device presented us with a mtu size that does not make
>>>>>>> sense" would point to failing, "we cannot work with the mtu size that
>>>>>>> the device presented us" would point to not negotiating the feature.
>>>>>>>>>              }
>>>>>>>>>
>>>>>>>>>              return 0;
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> And the spec says:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> The driver MUST follow this sequence to initialize a device:
>>>>>>>>> 1. Reset the device.
>>>>>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>>>>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
>>>>>>>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
>>>>>>>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
>>>>>>>>> fields to check that it can support the device before accepting it.
>>>>>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
>>>>>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
>>>>>>>>> support our subset of features and the device is unusable.
>>>>>>>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
>>>>>>>>> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
>>>>>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Item 4 on the list explicitly allows reading config space before
>>>>>>>>> FEATURES_OK.
>>>>>>>>>
>>>>>>>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
>>>>>>>> So this probably need some clarification. "is set" is used many times in
>>>>>>>> the spec that has different implications.
>>>>>>> Before FEATURES_OK is set by the driver, I guess it means "the device
>>>>>>> has offered the feature";
>>>>>> For me this part is ok since it clarify that it's the driver that set
>>>>>> the bit.
>>>>>>
>>>>>>
>>>>>>> during normal usage, it means "the feature
>>>>>>> has been negotiated".
>>>>>> /?
>>>>>>
>>>>>> It looks to me the feature negotiation is done only after device set
>>>>>> FEATURES_OK, or FEATURES_OK could be read from device status?
>>>>> I'd consider feature negotiation done when the driver reads FEATURES_OK
>>>>> back from the status.
>>>> I agree.
>>>>
>>>>
>>>>>>>      (This is a bit fuzzy for legacy mode.)
>>>>> ...because legacy does not have FEATURES_OK.
>>>>>> The problem is the MTU description for example:
>>>>>>
>>>>>> "The following driver-read-only field, mtu only exists if
>>>>>> VIRTIO_NET_F_MTU is set."
>>>>>>
>>>>>> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".
>>>>> "offered by the device"? I don't think it should 'disappear' from the
>>>>> config space if the driver won't use it. (Same for other config space
>>>>> fields that are tied to feature bits.)
>>>> But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks
>>>> to according to the spec there will be no mtu field.
>>> I think so, yes.
>>>
>>>> And a more interesting case is VIRTIO_NET_F_MQ is not offered but
>>>> VIRTIO_NET_F_MTU offered. To me, it means we don't have
>>>> max_virtqueue_pairs but it's not how the driver is wrote today.
>>> That would be a bug, but it seems to me that the virtio-net driver
>>> reads max_virtqueue_pairs conditionally and handles absence of the
>>> feature correctly?
>>
>> Yes, see the avove codes:
>>
>>          if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>                  int mtu = virtio_cread16(vdev,
>>                                           offsetof(struct virtio_net_config,
>>                                                    mtu));
>>                  if (mtu < MIN_MTU)
>>                          __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>          }
>>
>> So it's probably too late to fix the driver.
>>
> Confused. What is wrong with the above? It never reads the
> field unless the feature has been offered by device.


So the spec said:

"

The following driver-read-only field, max_virtqueue_pairs only exists if 
VIRTIO_NET_F_MQ is set.

"

If I read this correctly, there will be no max_virtqueue_pairs field if 
the VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() 
violates what spec said.

Thanks


>
>
>>>>>> Otherwise readers (at least for me), may think the MTU is only valid
>>>>>> if driver set the bit.
>>>>> I think it would still be 'valid' in the sense that it exists and has
>>>>> some value in there filled in by the device, but a driver reading it
>>>>> without negotiating the feature would be buggy. (Like in the kernel
>>>>> code above; the kernel not liking the value does not make the field
>>>>> invalid.)
>>>> See Michael's reply, the spec allows read the config before setting
>>>> features.
>>> Yes, the period prior to finishing negotiation is obviously special.
>>>
>>>>> Maybe a statement covering everything would be:
>>>>>
>>>>> "The following driver-read-only field mtu only exists if the device
>>>>> offers VIRTIO_NET_F_MTU and may be read by the driver during feature
>>>>> negotiation and after VIRTIO_NET_F_MTU has been successfully
>>>>> negotiated."
>>>>>>> Should we add a wording clarification to the spec?
>>>>>> I think so.
>>>>> Some clarification would be needed for each field that depends on a
>>>>> feature; that would be quite verbose. Maybe we can get away with a
>>>>> clarifying statement?
>>>>>
>>>>> "Some config space fields may depend on a certain feature. In that
>>>>> case, the field exits if the device has offered the corresponding
>>>>> feature,
>>>> So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config
>>>> will look like:
>>>>
>>>> struct virtio_net_config {
>>>>            u8 mac[6];
>>>>            le16 status;
>>>>            le16 mtu;
>>>> };
>>>>
>>> I agree.
>>
>> So consider it's probably too late to fix the driver which assumes some
>> field are always persent, it looks to me need fix the spec do declare the
>> fields are always existing instead.
>>
>>
>>>>>     and may be read by the driver during feature negotiation, and
>>>>> accessed by the driver after the feature has been successfully
>>>>> negotiated. A shorthand for this is a statement that a field only
>>>>> exists if a certain feature bit is set."
>>>> I'm not sure using "shorthand" is good for the spec, at least we can
>>>> limit the its scope only to the configuration space part.
>>> Maybe "a shorthand expression"?
>>
>> So the questions is should we use this for all over the spec or it will be
>> only used in this speicifc part (device configuration).
>>
>> Thanks
>>

Michael S. Tsirkin Feb. 28, 2021, 9:25 p.m. UTC | #48

On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:
> 
> On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:
> > On Thu, Feb 25, 2021 at 12:36:07PM +0800, Jason Wang wrote:
> > > On 2021/2/24 7:12 下午, Cornelia Huck wrote:
> > > > On Wed, 24 Feb 2021 17:29:07 +0800
> > > > Jason Wang <[e-mail]> wrote:
> > > > 
> > > > > On 2021/2/23 6:58 下午, Cornelia Huck wrote:
> > > > > > On Tue, 23 Feb 2021 18:31:07 +0800
> > > > > > Jason Wang <[e-mail]> wrote:
> > > > > > > On 2021/2/23 6:04 下午, Cornelia Huck wrote:
> > > > > > > > On Tue, 23 Feb 2021 17:46:20 +0800
> > > > > > > > Jason Wang <[e-mail]> wrote:
> > > > > > > > > On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
> > > > > > > > > > On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
> > > > > > > > > > > On 2/21/2021 8:14 PM, Jason Wang wrote:
> > > > > > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > > > > > space before virtio features are set.
> > > > > > > > > > > > This looks like a spec violation:
> > > > > > > > > > > > 
> > > > > > > > > > > > "
> > > > > > > > > > > > 
> > > > > > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > > > > > VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
> > > > > > > > > > > > driver to use.
> > > > > > > > > > > > "
> > > > > > > > > > > > 
> > > > > > > > > > > > Do we really want to workaround this?
> > > > > > > > > > > Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
> > > > > > > > > > > 
> > > > > > > > > > > I think the point is, since there's legacy guest we'd have to support, this
> > > > > > > > > > > host side workaround is unavoidable. Although I agree the violating driver
> > > > > > > > > > > should be fixed (yes, it's in today's upstream kernel which exists for a
> > > > > > > > > > > while now).
> > > > > > > > > > Oh  you are right:
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > static int virtnet_validate(struct virtio_device *vdev)
> > > > > > > > > > {
> > > > > > > > > >              if (!vdev->config->get) {
> > > > > > > > > >                      dev_err(&vdev->dev, "%s failure: config access disabled\n",
> > > > > > > > > >                              __func__);
> > > > > > > > > >                      return -EINVAL;
> > > > > > > > > >              }
> > > > > > > > > > 
> > > > > > > > > >              if (!virtnet_validate_features(vdev))
> > > > > > > > > >                      return -EINVAL;
> > > > > > > > > > 
> > > > > > > > > >              if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> > > > > > > > > >                      int mtu = virtio_cread16(vdev,
> > > > > > > > > >                                               offsetof(struct virtio_net_config,
> > > > > > > > > >                                                        mtu));
> > > > > > > > > >                      if (mtu < MIN_MTU)
> > > > > > > > > >                              __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
> > > > > > > > > I wonder why not simply fail here?
> > > > > > > > I think both failing or not accepting the feature can be argued to make
> > > > > > > > sense: "the device presented us with a mtu size that does not make
> > > > > > > > sense" would point to failing, "we cannot work with the mtu size that
> > > > > > > > the device presented us" would point to not negotiating the feature.
> > > > > > > > > >              }
> > > > > > > > > > 
> > > > > > > > > >              return 0;
> > > > > > > > > > }
> > > > > > > > > > 
> > > > > > > > > > And the spec says:
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > > > > 1. Reset the device.
> > > > > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > > > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> > > > > > > > > > 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
> > > > > > > > > > device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
> > > > > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
> > > > > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
> > > > > > > > > > support our subset of features and the device is unusable.
> > > > > > > > > > 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
> > > > > > > > > > reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
> > > > > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > Item 4 on the list explicitly allows reading config space before
> > > > > > > > > > FEATURES_OK.
> > > > > > > > > > 
> > > > > > > > > > I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
> > > > > > > > > So this probably need some clarification. "is set" is used many times in
> > > > > > > > > the spec that has different implications.
> > > > > > > > Before FEATURES_OK is set by the driver, I guess it means "the device
> > > > > > > > has offered the feature";
> > > > > > > For me this part is ok since it clarify that it's the driver that set
> > > > > > > the bit.
> > > > > > > 
> > > > > > > 
> > > > > > > > during normal usage, it means "the feature
> > > > > > > > has been negotiated".
> > > > > > > /?
> > > > > > > 
> > > > > > > It looks to me the feature negotiation is done only after device set
> > > > > > > FEATURES_OK, or FEATURES_OK could be read from device status?
> > > > > > I'd consider feature negotiation done when the driver reads FEATURES_OK
> > > > > > back from the status.
> > > > > I agree.
> > > > > 
> > > > > 
> > > > > > > >      (This is a bit fuzzy for legacy mode.)
> > > > > > ...because legacy does not have FEATURES_OK.
> > > > > > > The problem is the MTU description for example:
> > > > > > > 
> > > > > > > "The following driver-read-only field, mtu only exists if
> > > > > > > VIRTIO_NET_F_MTU is set."
> > > > > > > 
> > > > > > > It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".
> > > > > > "offered by the device"? I don't think it should 'disappear' from the
> > > > > > config space if the driver won't use it. (Same for other config space
> > > > > > fields that are tied to feature bits.)
> > > > > But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks
> > > > > to according to the spec there will be no mtu field.
> > > > I think so, yes.
> > > > 
> > > > > And a more interesting case is VIRTIO_NET_F_MQ is not offered but
> > > > > VIRTIO_NET_F_MTU offered. To me, it means we don't have
> > > > > max_virtqueue_pairs but it's not how the driver is wrote today.
> > > > That would be a bug, but it seems to me that the virtio-net driver
> > > > reads max_virtqueue_pairs conditionally and handles absence of the
> > > > feature correctly?
> > > 
> > > Yes, see the avove codes:
> > > 
> > >          if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> > >                  int mtu = virtio_cread16(vdev,
> > >                                           offsetof(struct virtio_net_config,
> > >                                                    mtu));
> > >                  if (mtu < MIN_MTU)
> > >                          __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
> > >          }
> > > 
> > > So it's probably too late to fix the driver.
> > > 
> > Confused. What is wrong with the above? It never reads the
> > field unless the feature has been offered by device.
> 
> 
> So the spec said:
> 
> "
> 
> The following driver-read-only field, max_virtqueue_pairs only exists if
> VIRTIO_NET_F_MQ is set.
> 
> "
> 
> If I read this correctly, there will be no max_virtqueue_pairs field if the
> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
> what spec said.
> 
> Thanks

I think that's a misunderstanding. This text was never intended to
imply that field offsets change beased on feature bits.
We had this pain with legacy and we never wanted to go back there.

This merely implies that without VIRTIO_NET_F_MQ the field
should not be accessed. Exists in the sense "is accessible to driver".

Let's just clarify that in the spec, job done.




> 
> > 
> > 
> > > > > > > Otherwise readers (at least for me), may think the MTU is only valid
> > > > > > > if driver set the bit.
> > > > > > I think it would still be 'valid' in the sense that it exists and has
> > > > > > some value in there filled in by the device, but a driver reading it
> > > > > > without negotiating the feature would be buggy. (Like in the kernel
> > > > > > code above; the kernel not liking the value does not make the field
> > > > > > invalid.)
> > > > > See Michael's reply, the spec allows read the config before setting
> > > > > features.
> > > > Yes, the period prior to finishing negotiation is obviously special.
> > > > 
> > > > > > Maybe a statement covering everything would be:
> > > > > > 
> > > > > > "The following driver-read-only field mtu only exists if the device
> > > > > > offers VIRTIO_NET_F_MTU and may be read by the driver during feature
> > > > > > negotiation and after VIRTIO_NET_F_MTU has been successfully
> > > > > > negotiated."
> > > > > > > > Should we add a wording clarification to the spec?
> > > > > > > I think so.
> > > > > > Some clarification would be needed for each field that depends on a
> > > > > > feature; that would be quite verbose. Maybe we can get away with a
> > > > > > clarifying statement?
> > > > > > 
> > > > > > "Some config space fields may depend on a certain feature. In that
> > > > > > case, the field exits if the device has offered the corresponding
> > > > > > feature,
> > > > > So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config
> > > > > will look like:
> > > > > 
> > > > > struct virtio_net_config {
> > > > >            u8 mac[6];
> > > > >            le16 status;
> > > > >            le16 mtu;
> > > > > };
> > > > > 
> > > > I agree.
> > > 
> > > So consider it's probably too late to fix the driver which assumes some
> > > field are always persent, it looks to me need fix the spec do declare the
> > > fields are always existing instead.
> > > 
> > > 
> > > > > >     and may be read by the driver during feature negotiation, and
> > > > > > accessed by the driver after the feature has been successfully
> > > > > > negotiated. A shorthand for this is a statement that a field only
> > > > > > exists if a certain feature bit is set."
> > > > > I'm not sure using "shorthand" is good for the spec, at least we can
> > > > > limit the its scope only to the configuration space part.
> > > > Maybe "a shorthand expression"?
> > > 
> > > So the questions is should we use this for all over the spec or it will be
> > > only used in this speicifc part (device configuration).
> > > 
> > > Thanks
> > >

Michael S. Tsirkin Feb. 28, 2021, 9:27 p.m. UTC | #49

On Thu, Feb 25, 2021 at 04:56:42PM -0800, Si-Wei Liu wrote:
> 
> Hi Michael,
> 
> Are you okay to live without this ioctl for now? I think QEMU is the one
> that needs to be fixed and will have to be made legacy guest aware. I think
> the kernel can just honor the feature negotiation result done by QEMU and do
> as what's told to.Will you agree?
> 
> If it's fine, I would proceed to reverting commit fe36cbe067 and related
> code in question from the kernel.
> 
> Thanks,
> -Siwei


Not really, I don't see why that's a good idea.  fe36cbe067 is the code
checking MTU before FEATURES_OK. Spec explicitly allows that.

Michael S. Tsirkin Feb. 28, 2021, 9:28 p.m. UTC | #50

On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> 
> 
> On 2/23/2021 9:04 PM, Michael S. Tsirkin wrote:
> > On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
> > > 
> > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > 
> > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > space before virtio features are set.
> > > > > > > > This looks like a spec violation:
> > > > > > > > 
> > > > > > > > "
> > > > > > > > 
> > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > set.
> > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > "
> > > > > > > > 
> > > > > > > > Do we really want to workaround this?
> > > > > > > > 
> > > > > > > > Thanks
> > > > > > > And also:
> > > > > > > 
> > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > 1. Reset the device.
> > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > device.
> > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > understood by the OS and driver to the
> > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > the device-specific configuration
> > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > feature bits after this step.
> > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > otherwise, the device does not
> > > > > > > support our subset of features and the device is unusable.
> > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > for the device, optional per-bus setup,
> > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > space, and population of virtqueues.
> > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > 
> > > > > > > 
> > > > > > > so accessing config space before FEATURES_OK is a spec violation, right?
> > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > thought the legacy guest still needs to be supported.
> > > > > > 
> > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > issue where this discrepancy is introduced to virtnet_validate() (since
> > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > 
> > > > > > -Siwei
> > > > > I think it's a bug to read config space in validate, we should move it to
> > > > > virtnet_probe().
> > > > > 
> > > > > Thanks
> > > > I take it back, reading but not writing seems to be explicitly allowed by spec.
> > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > the best way to handle this.
> > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
> > > guest? Supposedly only config space write access needs to be guarded before
> > > setting FEATURES_OK.
> > > 
> > > -Siwie
> > Detecting it isn't enough though, we will need a new ioctl to notify
> > the kernel that it's a legacy guest. Ugh :(
> Well, although I think adding an ioctl is doable, may I know what the use
> case there will be for kernel to leverage such info directly? Is there a
> case QEMU can't do with dedicate ioctls later if there's indeed
> differentiation (legacy v.s. modern) needed?
> 
> One of the reason I asked is if this ioctl becomes a mandate for vhost-vdpa
> kernel. QEMU would reject initialize vhost-vdpa if doesn't see this ioctl
> coming?

Only on BE hosts or guests I think. With LE host and guest legacy and
modern behave the same so ioctl isn't needed.

> If it's optional, suppose the kernel may need it only when it becomes
> necessary?
> 
> Thanks,
> -Siwei

Michael S. Tsirkin Feb. 28, 2021, 9:30 p.m. UTC | #51

On Wed, Feb 24, 2021 at 05:30:37PM +0800, Jason Wang wrote:
> 
> On 2021/2/24 4:43 下午, Michael S. Tsirkin wrote:
> > On Wed, Feb 24, 2021 at 04:26:43PM +0800, Jason Wang wrote:
> > >      Basically on first guest access QEMU would tell kernel whether
> > >      guest is using the legacy or the modern interface.
> > >      E.g. virtio_pci_config_read/virtio_pci_config_write will call ioctl(ENABLE_LEGACY, 1)
> > >      while virtio_pci_common_read will call ioctl(ENABLE_LEGACY, 0)
> > > 
> > > 
> > > But this trick work only for PCI I think?
> > > 
> > > Thanks
> > ccw has a revision it can check. mmio does not have transitional devices
> > at all.
> 
> 
> Ok, then we can do the workaround in the qemu, isn't it?
> 
> Thanks

which one do you mean?

Michael S. Tsirkin Feb. 28, 2021, 9:34 p.m. UTC | #52

On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > Detecting it isn't enough though, we will need a new ioctl to notify
> > the kernel that it's a legacy guest. Ugh :(
> Well, although I think adding an ioctl is doable, may I know what the use
> case there will be for kernel to leverage such info directly? Is there a
> case QEMU can't do with dedicate ioctls later if there's indeed
> differentiation (legacy v.s. modern) needed?

BTW a good API could be

#define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
#define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)

we did it per vring but maybe that was a mistake ...

Jason Wang March 1, 2021, 3:51 a.m. UTC | #53

On 2021/3/1 5:25 上午, Michael S. Tsirkin wrote:
> On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:
>> On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:
>>> On Thu, Feb 25, 2021 at 12:36:07PM +0800, Jason Wang wrote:
>>>> On 2021/2/24 7:12 下午, Cornelia Huck wrote:
>>>>> On Wed, 24 Feb 2021 17:29:07 +0800
>>>>> Jason Wang <[e-mail]> wrote:
>>>>>
>>>>>> On 2021/2/23 6:58 下午, Cornelia Huck wrote:
>>>>>>> On Tue, 23 Feb 2021 18:31:07 +0800
>>>>>>> Jason Wang <[e-mail]> wrote:
>>>>>>>> On 2021/2/23 6:04 下午, Cornelia Huck wrote:
>>>>>>>>> On Tue, 23 Feb 2021 17:46:20 +0800
>>>>>>>>> Jason Wang <[e-mail]> wrote:
>>>>>>>>>> On 2021/2/23 下午5:25, Michael S. Tsirkin wrote:
>>>>>>>>>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:
>>>>>>>>>>>> On 2/21/2021 8:14 PM, Jason Wang wrote:
>>>>>>>>>>>>> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>>>>>>>>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>>>>>>>>>>>>>> for legacy") made an exception for legacy guests to reset
>>>>>>>>>>>>>> features to 0, when config space is accessed before features
>>>>>>>>>>>>>> are set. We should relieve the verify_min_features() check
>>>>>>>>>>>>>> and allow features reset to 0 for this case.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> It's worth noting that not just legacy guests could access
>>>>>>>>>>>>>> config space before features are set. For instance, when
>>>>>>>>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>>>>>>>>>>>>>> will try to access and validate the MTU present in the config
>>>>>>>>>>>>>> space before virtio features are set.
>>>>>>>>>>>>> This looks like a spec violation:
>>>>>>>>>>>>>
>>>>>>>>>>>>> "
>>>>>>>>>>>>>
>>>>>>>>>>>>> The following driver-read-only field, mtu only exists if
>>>>>>>>>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the
>>>>>>>>>>>>> driver to use.
>>>>>>>>>>>>> "
>>>>>>>>>>>>>
>>>>>>>>>>>>> Do we really want to workaround this?
>>>>>>>>>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?
>>>>>>>>>>>>
>>>>>>>>>>>> I think the point is, since there's legacy guest we'd have to support, this
>>>>>>>>>>>> host side workaround is unavoidable. Although I agree the violating driver
>>>>>>>>>>>> should be fixed (yes, it's in today's upstream kernel which exists for a
>>>>>>>>>>>> while now).
>>>>>>>>>>> Oh  you are right:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> static int virtnet_validate(struct virtio_device *vdev)
>>>>>>>>>>> {
>>>>>>>>>>>               if (!vdev->config->get) {
>>>>>>>>>>>                       dev_err(&vdev->dev, "%s failure: config access disabled\n",
>>>>>>>>>>>                               __func__);
>>>>>>>>>>>                       return -EINVAL;
>>>>>>>>>>>               }
>>>>>>>>>>>
>>>>>>>>>>>               if (!virtnet_validate_features(vdev))
>>>>>>>>>>>                       return -EINVAL;
>>>>>>>>>>>
>>>>>>>>>>>               if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>>>>>>>>>>                       int mtu = virtio_cread16(vdev,
>>>>>>>>>>>                                                offsetof(struct virtio_net_config,
>>>>>>>>>>>                                                         mtu));
>>>>>>>>>>>                       if (mtu < MIN_MTU)
>>>>>>>>>>>                               __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>>>>>>>>> I wonder why not simply fail here?
>>>>>>>>> I think both failing or not accepting the feature can be argued to make
>>>>>>>>> sense: "the device presented us with a mtu size that does not make
>>>>>>>>> sense" would point to failing, "we cannot work with the mtu size that
>>>>>>>>> the device presented us" would point to not negotiating the feature.
>>>>>>>>>>>               }
>>>>>>>>>>>
>>>>>>>>>>>               return 0;
>>>>>>>>>>> }
>>>>>>>>>>>
>>>>>>>>>>> And the spec says:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> The driver MUST follow this sequence to initialize a device:
>>>>>>>>>>> 1. Reset the device.
>>>>>>>>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
>>>>>>>>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
>>>>>>>>>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
>>>>>>>>>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
>>>>>>>>>>> fields to check that it can support the device before accepting it.
>>>>>>>>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
>>>>>>>>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
>>>>>>>>>>> support our subset of features and the device is unusable.
>>>>>>>>>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
>>>>>>>>>>> reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
>>>>>>>>>>> 8. Set the DRIVER_OK status bit. At this point the device is “live”.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Item 4 on the list explicitly allows reading config space before
>>>>>>>>>>> FEATURES_OK.
>>>>>>>>>>>
>>>>>>>>>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features".
>>>>>>>>>> So this probably need some clarification. "is set" is used many times in
>>>>>>>>>> the spec that has different implications.
>>>>>>>>> Before FEATURES_OK is set by the driver, I guess it means "the device
>>>>>>>>> has offered the feature";
>>>>>>>> For me this part is ok since it clarify that it's the driver that set
>>>>>>>> the bit.
>>>>>>>>
>>>>>>>>
>>>>>>>>> during normal usage, it means "the feature
>>>>>>>>> has been negotiated".
>>>>>>>> /?
>>>>>>>>
>>>>>>>> It looks to me the feature negotiation is done only after device set
>>>>>>>> FEATURES_OK, or FEATURES_OK could be read from device status?
>>>>>>> I'd consider feature negotiation done when the driver reads FEATURES_OK
>>>>>>> back from the status.
>>>>>> I agree.
>>>>>>
>>>>>>
>>>>>>>>>       (This is a bit fuzzy for legacy mode.)
>>>>>>> ...because legacy does not have FEATURES_OK.
>>>>>>>> The problem is the MTU description for example:
>>>>>>>>
>>>>>>>> "The following driver-read-only field, mtu only exists if
>>>>>>>> VIRTIO_NET_F_MTU is set."
>>>>>>>>
>>>>>>>> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device".
>>>>>>> "offered by the device"? I don't think it should 'disappear' from the
>>>>>>> config space if the driver won't use it. (Same for other config space
>>>>>>> fields that are tied to feature bits.)
>>>>>> But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks
>>>>>> to according to the spec there will be no mtu field.
>>>>> I think so, yes.
>>>>>
>>>>>> And a more interesting case is VIRTIO_NET_F_MQ is not offered but
>>>>>> VIRTIO_NET_F_MTU offered. To me, it means we don't have
>>>>>> max_virtqueue_pairs but it's not how the driver is wrote today.
>>>>> That would be a bug, but it seems to me that the virtio-net driver
>>>>> reads max_virtqueue_pairs conditionally and handles absence of the
>>>>> feature correctly?
>>>> Yes, see the avove codes:
>>>>
>>>>           if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>>>>                   int mtu = virtio_cread16(vdev,
>>>>                                            offsetof(struct virtio_net_config,
>>>>                                                     mtu));
>>>>                   if (mtu < MIN_MTU)
>>>>                           __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
>>>>           }
>>>>
>>>> So it's probably too late to fix the driver.
>>>>
>>> Confused. What is wrong with the above? It never reads the
>>> field unless the feature has been offered by device.
>>
>> So the spec said:
>>
>> "
>>
>> The following driver-read-only field, max_virtqueue_pairs only exists if
>> VIRTIO_NET_F_MQ is set.
>>
>> "
>>
>> If I read this correctly, there will be no max_virtqueue_pairs field if the
>> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
>> what spec said.
>>
>> Thanks
> I think that's a misunderstanding. This text was never intended to
> imply that field offsets change beased on feature bits.
> We had this pain with legacy and we never wanted to go back there.
>
> This merely implies that without VIRTIO_NET_F_MQ the field
> should not be accessed. Exists in the sense "is accessible to driver".
>
> Let's just clarify that in the spec, job done.


Ok, agree. That will make things more eaiser.

Thanks


>
>
>
>
>>>
>>>>>>>> Otherwise readers (at least for me), may think the MTU is only valid
>>>>>>>> if driver set the bit.
>>>>>>> I think it would still be 'valid' in the sense that it exists and has
>>>>>>> some value in there filled in by the device, but a driver reading it
>>>>>>> without negotiating the feature would be buggy. (Like in the kernel
>>>>>>> code above; the kernel not liking the value does not make the field
>>>>>>> invalid.)
>>>>>> See Michael's reply, the spec allows read the config before setting
>>>>>> features.
>>>>> Yes, the period prior to finishing negotiation is obviously special.
>>>>>
>>>>>>> Maybe a statement covering everything would be:
>>>>>>>
>>>>>>> "The following driver-read-only field mtu only exists if the device
>>>>>>> offers VIRTIO_NET_F_MTU and may be read by the driver during feature
>>>>>>> negotiation and after VIRTIO_NET_F_MTU has been successfully
>>>>>>> negotiated."
>>>>>>>>> Should we add a wording clarification to the spec?
>>>>>>>> I think so.
>>>>>>> Some clarification would be needed for each field that depends on a
>>>>>>> feature; that would be quite verbose. Maybe we can get away with a
>>>>>>> clarifying statement?
>>>>>>>
>>>>>>> "Some config space fields may depend on a certain feature. In that
>>>>>>> case, the field exits if the device has offered the corresponding
>>>>>>> feature,
>>>>>> So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config
>>>>>> will look like:
>>>>>>
>>>>>> struct virtio_net_config {
>>>>>>             u8 mac[6];
>>>>>>             le16 status;
>>>>>>             le16 mtu;
>>>>>> };
>>>>>>
>>>>> I agree.
>>>> So consider it's probably too late to fix the driver which assumes some
>>>> field are always persent, it looks to me need fix the spec do declare the
>>>> fields are always existing instead.
>>>>
>>>>
>>>>>>>      and may be read by the driver during feature negotiation, and
>>>>>>> accessed by the driver after the feature has been successfully
>>>>>>> negotiated. A shorthand for this is a statement that a field only
>>>>>>> exists if a certain feature bit is set."
>>>>>> I'm not sure using "shorthand" is good for the spec, at least we can
>>>>>> limit the its scope only to the configuration space part.
>>>>> Maybe "a shorthand expression"?
>>>> So the questions is should we use this for all over the spec or it will be
>>>> only used in this speicifc part (device configuration).
>>>>
>>>> Thanks
>>>>

Jason Wang March 1, 2021, 3:53 a.m. UTC | #54

On 2021/3/1 5:30 上午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 05:30:37PM +0800, Jason Wang wrote:
>> On 2021/2/24 4:43 下午, Michael S. Tsirkin wrote:
>>> On Wed, Feb 24, 2021 at 04:26:43PM +0800, Jason Wang wrote:
>>>>       Basically on first guest access QEMU would tell kernel whether
>>>>       guest is using the legacy or the modern interface.
>>>>       E.g. virtio_pci_config_read/virtio_pci_config_write will call ioctl(ENABLE_LEGACY, 1)
>>>>       while virtio_pci_common_read will call ioctl(ENABLE_LEGACY, 0)
>>>>
>>>>
>>>> But this trick work only for PCI I think?
>>>>
>>>> Thanks
>>> ccw has a revision it can check. mmio does not have transitional devices
>>> at all.
>>
>> Ok, then we can do the workaround in the qemu, isn't it?
>>
>> Thanks
> which one do you mean?


I meant the workaround that is done by 452639a64ad8 ("vdpa: make sure 
set_features is invoked for legacy").

Thanks


>

Jason Wang March 1, 2021, 3:56 a.m. UTC | #55

On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>> the kernel that it's a legacy guest. Ugh :(
>> Well, although I think adding an ioctl is doable, may I know what the use
>> case there will be for kernel to leverage such info directly? Is there a
>> case QEMU can't do with dedicate ioctls later if there's indeed
>> differentiation (legacy v.s. modern) needed?
> BTW a good API could be
>
> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>
> we did it per vring but maybe that was a mistake ...


Actually, I wonder whether it's good time to just not support legacy 
driver for vDPA. Consider:

1) It's definition is no-normative
2) A lot of budren of codes

So qemu can still present the legacy device since the config space or 
other stuffs that is presented by vhost-vDPA is not expected to be 
accessed by guest directly. Qemu can do the endian conversion when 
necessary in this case?

Thanks


>

Si-Wei Liu March 1, 2021, 6:08 p.m. UTC | #56

On 2/28/2021 1:27 PM, Michael S. Tsirkin wrote:
> On Thu, Feb 25, 2021 at 04:56:42PM -0800, Si-Wei Liu wrote:
>> Hi Michael,
>>
>> Are you okay to live without this ioctl for now? I think QEMU is the one
>> that needs to be fixed and will have to be made legacy guest aware. I think
>> the kernel can just honor the feature negotiation result done by QEMU and do
>> as what's told to.Will you agree?
>>
>> If it's fine, I would proceed to reverting commit fe36cbe067 and related
>> code in question from the kernel.
>>
>> Thanks,
>> -Siwei
>
> Not really, I don't see why that's a good idea.  fe36cbe067 is the code
> checking MTU before FEATURES_OK. Spec explicitly allows that.
>
Alright, but what I meant was this commit
452639a64ad8 ("vdpa: make sure set_features is invoked for legacy").

But I got why you need it in another email (for BE host/guest).

-Siwei

Michael S. Tsirkin March 2, 2021, 9:47 a.m. UTC | #57

On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> 
> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > the kernel that it's a legacy guest. Ugh :(
> > > Well, although I think adding an ioctl is doable, may I know what the use
> > > case there will be for kernel to leverage such info directly? Is there a
> > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > differentiation (legacy v.s. modern) needed?
> > BTW a good API could be
> > 
> > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > 
> > we did it per vring but maybe that was a mistake ...
> 
> 
> Actually, I wonder whether it's good time to just not support legacy driver
> for vDPA. Consider:
> 
> 1) It's definition is no-normative
> 2) A lot of budren of codes
> 
> So qemu can still present the legacy device since the config space or other
> stuffs that is presented by vhost-vDPA is not expected to be accessed by
> guest directly. Qemu can do the endian conversion when necessary in this
> case?
> 
> Thanks
> 

Overall I would be fine with this approach but we need to avoid breaking
working userspace, qemu releases with vdpa support are out there and
seem to work for people. Any changes need to take that into account
and document compatibility concerns. I note that any hardware
implementation is already broken for legacy except on platforms with
strong ordering which might be helpful in reducing the scope.

Jason Wang March 2, 2021, 10:53 a.m. UTC | #58

On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>> the kernel that it's a legacy guest. Ugh :(
>>>> Well, although I think adding an ioctl is doable, may I know what the use
>>>> case there will be for kernel to leverage such info directly? Is there a
>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>> differentiation (legacy v.s. modern) needed?
>>> BTW a good API could be
>>>
>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>
>>> we did it per vring but maybe that was a mistake ...
>>
>> Actually, I wonder whether it's good time to just not support legacy driver
>> for vDPA. Consider:
>>
>> 1) It's definition is no-normative
>> 2) A lot of budren of codes
>>
>> So qemu can still present the legacy device since the config space or other
>> stuffs that is presented by vhost-vDPA is not expected to be accessed by
>> guest directly. Qemu can do the endian conversion when necessary in this
>> case?
>>
>> Thanks
>>
> Overall I would be fine with this approach but we need to avoid breaking
> working userspace, qemu releases with vdpa support are out there and
> seem to work for people. Any changes need to take that into account
> and document compatibility concerns.


Agree, let me check.


>   I note that any hardware
> implementation is already broken for legacy except on platforms with
> strong ordering which might be helpful in reducing the scope.


Yes.

Thanks


>
>

Cornelia Huck March 2, 2021, 12:08 p.m. UTC | #59

On Mon, 1 Mar 2021 11:51:08 +0800
Jason Wang <[e-mail]> wrote:

> On 2021/3/1 5:25 上午, Michael S. Tsirkin wrote:
> > On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:  
> >> On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:  

> >>> Confused. What is wrong with the above? It never reads the
> >>> field unless the feature has been offered by device.  
> >>
> >> So the spec said:
> >>
> >> "
> >>
> >> The following driver-read-only field, max_virtqueue_pairs only exists if
> >> VIRTIO_NET_F_MQ is set.
> >>
> >> "
> >>
> >> If I read this correctly, there will be no max_virtqueue_pairs field if the
> >> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
> >> what spec said.
> >>
> >> Thanks  
> > I think that's a misunderstanding. This text was never intended to
> > imply that field offsets change beased on feature bits.
> > We had this pain with legacy and we never wanted to go back there.
> >
> > This merely implies that without VIRTIO_NET_F_MQ the field
> > should not be accessed. Exists in the sense "is accessible to driver".
> >
> > Let's just clarify that in the spec, job done.  
> 
> 
> Ok, agree. That will make things more eaiser.

Yes, that makes much more sense.

What about adding the following to the "Basic Facilities of a Virtio
Device/Device Configuration Space" section of the spec:

"If an optional configuration field does not exist, the corresponding
space is still present, but reserved."

(Do we need to specify what a device needs to do if the driver tries to
access a non-existing field? We cannot really make assumptions about
config space accesses; virtio-ccw can just copy a chunk of config space
that contains non-existing fields... I guess the device could ignore
writes and return zeroes on read?)

I've opened https://github.com/oasis-tcs/virtio-spec/issues/98 for the
spec issues.

Cornelia Huck March 3, 2021, 8:29 a.m. UTC | #60

On Wed, 3 Mar 2021 12:01:01 +0800
Jason Wang <[e-mail]> wrote:

> On 2021/3/2 8:08 下午, Cornelia Huck wrote:
> > On Mon, 1 Mar 2021 11:51:08 +0800
> > Jason Wang <[e-mail]> wrote:
> >  
> >> On 2021/3/1 5:25 上午, Michael S. Tsirkin wrote:  
> >>> On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:  
> >>>> On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:  
> >>>>> Confused. What is wrong with the above? It never reads the
> >>>>> field unless the feature has been offered by device.  
> >>>> So the spec said:
> >>>>
> >>>> "
> >>>>
> >>>> The following driver-read-only field, max_virtqueue_pairs only exists if
> >>>> VIRTIO_NET_F_MQ is set.
> >>>>
> >>>> "
> >>>>
> >>>> If I read this correctly, there will be no max_virtqueue_pairs field if the
> >>>> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
> >>>> what spec said.
> >>>>
> >>>> Thanks  
> >>> I think that's a misunderstanding. This text was never intended to
> >>> imply that field offsets change beased on feature bits.
> >>> We had this pain with legacy and we never wanted to go back there.
> >>>
> >>> This merely implies that without VIRTIO_NET_F_MQ the field
> >>> should not be accessed. Exists in the sense "is accessible to driver".
> >>>
> >>> Let's just clarify that in the spec, job done.  
> >>
> >> Ok, agree. That will make things more eaiser.  
> > Yes, that makes much more sense.
> >
> > What about adding the following to the "Basic Facilities of a Virtio
> > Device/Device Configuration Space" section of the spec:
> >
> > "If an optional configuration field does not exist, the corresponding
> > space is still present, but reserved."  
> 
> 
> This became interesting after re-reading some of the qemu codes.
> 
> E.g in virtio-net.c we had:
> 
> *static VirtIOFeature feature_sizes[] = {
>      {.flags = 1ULL << VIRTIO_NET_F_MAC,
>       .end = endof(struct virtio_net_config, mac)},
>      {.flags = 1ULL << VIRTIO_NET_F_STATUS,
>       .end = endof(struct virtio_net_config, status)},
>      {.flags = 1ULL << VIRTIO_NET_F_MQ,
>       .end = endof(struct virtio_net_config, max_virtqueue_pairs)},
>      {.flags = 1ULL << VIRTIO_NET_F_MTU,
>       .end = endof(struct virtio_net_config, mtu)},
>      {.flags = 1ULL << VIRTIO_NET_F_SPEED_DUPLEX,
>       .end = endof(struct virtio_net_config, duplex)},
>      {.flags = (1ULL << VIRTIO_NET_F_RSS) | (1ULL << 
> VIRTIO_NET_F_HASH_REPORT),
>       .end = endof(struct virtio_net_config, supported_hash_types)},
>      {}
> };*
> 
> *It has a implict dependency chain. E.g MTU doesn't presnet if 
> DUPLEX/RSS is not offered ...
> *

But I think it covers everything up to the relevant field, no? So MTU
is included if we have the feature bit, even if we don't have
DUPLEX/RSS.

Given that a config space may be shorter (but must not collapse
non-existing fields), maybe a better wording would be:

"If an optional configuration field does not exist, the corresponding
space will still be present if it is not at the end of the
configuration space (i.e., further configuration fields exist.) This
implies that a given field, if it exists, is always at the same offset
from the beginning of the configuration space."


> >
> > (Do we need to specify what a device needs to do if the driver tries to
> > access a non-existing field? We cannot really make assumptions about
> > config space accesses; virtio-ccw can just copy a chunk of config space
> > that contains non-existing fields...  
> 
> 
> Right, it looks to me ccw doesn't depends on config len which is kind of 
> interesting. I think the answer depends on the implementation of both 
> transport and device:

(virtio-ccw is a bit odd, because channel I/O does not have the concept
of a config space and I needed to come up with something)

> 
> Let's take virtio-net-pci as an example, it fills status unconditionally 
> in virtio_net_set_config() even if VIRTIO_NET_F_STATUS is not 
> negotiated. So with the above feature_sizes:
> 
> 1) if one of the MQ, MTU, DUPLEX or RSS is implemented, driver can still 
> read status in this case
> 2) if none of the above four is negotied, driver can only read a 0xFF 
> (virtio_config_readb())
> 
> 
> > I guess the device could ignore
> > writes and return zeroes on read?)  
> 
> 
> So consider the above, it might be too late to fix/clarify that in the 
> spec on the expected behaviour of reading/writing non-exist fields.

We could still advise behaviour via SHOULD, though I'm not sure it adds
much at this point in time.

It really feels a bit odd that a driver can still read and write fields
for features that it did not negotiate, but I fear we're stuck with it.

> 
> Thanks
> 
> 
> >
> > I've opened https://github.com/oasis-tcs/virtio-spec/issues/98 for the
> > spec issues.
> >

Jason Wang March 4, 2021, 8:24 a.m. UTC | #61

On 2021/3/3 4:29 下午, Cornelia Huck wrote:
> On Wed, 3 Mar 2021 12:01:01 +0800
> Jason Wang <[e-mail]> wrote:
>
>> On 2021/3/2 8:08 下午, Cornelia Huck wrote:
>>> On Mon, 1 Mar 2021 11:51:08 +0800
>>> Jason Wang <[e-mail]> wrote:
>>>   
>>>> On 2021/3/1 5:25 上午, Michael S. Tsirkin wrote:
>>>>> On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:
>>>>>> On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:
>>>>>>> Confused. What is wrong with the above? It never reads the
>>>>>>> field unless the feature has been offered by device.
>>>>>> So the spec said:
>>>>>>
>>>>>> "
>>>>>>
>>>>>> The following driver-read-only field, max_virtqueue_pairs only exists if
>>>>>> VIRTIO_NET_F_MQ is set.
>>>>>>
>>>>>> "
>>>>>>
>>>>>> If I read this correctly, there will be no max_virtqueue_pairs field if the
>>>>>> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
>>>>>> what spec said.
>>>>>>
>>>>>> Thanks
>>>>> I think that's a misunderstanding. This text was never intended to
>>>>> imply that field offsets change beased on feature bits.
>>>>> We had this pain with legacy and we never wanted to go back there.
>>>>>
>>>>> This merely implies that without VIRTIO_NET_F_MQ the field
>>>>> should not be accessed. Exists in the sense "is accessible to driver".
>>>>>
>>>>> Let's just clarify that in the spec, job done.
>>>> Ok, agree. That will make things more eaiser.
>>> Yes, that makes much more sense.
>>>
>>> What about adding the following to the "Basic Facilities of a Virtio
>>> Device/Device Configuration Space" section of the spec:
>>>
>>> "If an optional configuration field does not exist, the corresponding
>>> space is still present, but reserved."
>>
>> This became interesting after re-reading some of the qemu codes.
>>
>> E.g in virtio-net.c we had:
>>
>> *static VirtIOFeature feature_sizes[] = {
>>       {.flags = 1ULL << VIRTIO_NET_F_MAC,
>>        .end = endof(struct virtio_net_config, mac)},
>>       {.flags = 1ULL << VIRTIO_NET_F_STATUS,
>>        .end = endof(struct virtio_net_config, status)},
>>       {.flags = 1ULL << VIRTIO_NET_F_MQ,
>>        .end = endof(struct virtio_net_config, max_virtqueue_pairs)},
>>       {.flags = 1ULL << VIRTIO_NET_F_MTU,
>>        .end = endof(struct virtio_net_config, mtu)},
>>       {.flags = 1ULL << VIRTIO_NET_F_SPEED_DUPLEX,
>>        .end = endof(struct virtio_net_config, duplex)},
>>       {.flags = (1ULL << VIRTIO_NET_F_RSS) | (1ULL <<
>> VIRTIO_NET_F_HASH_REPORT),
>>        .end = endof(struct virtio_net_config, supported_hash_types)},
>>       {}
>> };*
>>
>> *It has a implict dependency chain. E.g MTU doesn't presnet if
>> DUPLEX/RSS is not offered ...
>> *
> But I think it covers everything up to the relevant field, no? So MTU
> is included if we have the feature bit, even if we don't have
> DUPLEX/RSS.
>
> Given that a config space may be shorter (but must not collapse
> non-existing fields), maybe a better wording would be:
>
> "If an optional configuration field does not exist, the corresponding
> space will still be present if it is not at the end of the
> configuration space (i.e., further configuration fields exist.)


This should work but I think we need to define the end of configuration 
space first?


> This
> implies that a given field, if it exists, is always at the same offset
> from the beginning of the configuration space."
>
>
>>> (Do we need to specify what a device needs to do if the driver tries to
>>> access a non-existing field? We cannot really make assumptions about
>>> config space accesses; virtio-ccw can just copy a chunk of config space
>>> that contains non-existing fields...
>>
>> Right, it looks to me ccw doesn't depends on config len which is kind of
>> interesting. I think the answer depends on the implementation of both
>> transport and device:
> (virtio-ccw is a bit odd, because channel I/O does not have the concept
> of a config space and I needed to come up with something)


Ok.


>
>> Let's take virtio-net-pci as an example, it fills status unconditionally
>> in virtio_net_set_config() even if VIRTIO_NET_F_STATUS is not
>> negotiated. So with the above feature_sizes:
>>
>> 1) if one of the MQ, MTU, DUPLEX or RSS is implemented, driver can still
>> read status in this case
>> 2) if none of the above four is negotied, driver can only read a 0xFF
>> (virtio_config_readb())
>>
>>
>>> I guess the device could ignore
>>> writes and return zeroes on read?)
>>
>> So consider the above, it might be too late to fix/clarify that in the
>> spec on the expected behaviour of reading/writing non-exist fields.
> We could still advise behaviour via SHOULD, though I'm not sure it adds
> much at this point in time.
>
> It really feels a bit odd that a driver can still read and write fields
> for features that it did not negotiate, but I fear we're stuck with it.


Yes, since the device (at least virtio-pci) implment thing like this.

Thanks


>
>> Thanks
>>
>>
>>> I've opened https://github.com/oasis-tcs/virtio-spec/issues/98 for the
>>> spec issues.
>>>   
>
> ---------------------------------------------------------------------
> To unsubscribe, e-mail: [e-mail]
> For additional commands, e-mail: [e-mail]
>

Cornelia Huck March 4, 2021, 1:50 p.m. UTC | #62

On Thu, 4 Mar 2021 16:24:16 +0800
Jason Wang <[e-mail]> wrote:

> On 2021/3/3 4:29 下午, Cornelia Huck wrote:
> > On Wed, 3 Mar 2021 12:01:01 +0800
> > Jason Wang <[e-mail]> wrote:
> >  
> >> On 2021/3/2 8:08 下午, Cornelia Huck wrote:  
> >>> On Mon, 1 Mar 2021 11:51:08 +0800
> >>> Jason Wang <[e-mail]> wrote:
> >>>     
> >>>> On 2021/3/1 5:25 上午, Michael S. Tsirkin wrote:  
> >>>>> On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:  
> >>>>>> On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:  
> >>>>>>> Confused. What is wrong with the above? It never reads the
> >>>>>>> field unless the feature has been offered by device.  
> >>>>>> So the spec said:
> >>>>>>
> >>>>>> "
> >>>>>>
> >>>>>> The following driver-read-only field, max_virtqueue_pairs only exists if
> >>>>>> VIRTIO_NET_F_MQ is set.
> >>>>>>
> >>>>>> "
> >>>>>>
> >>>>>> If I read this correctly, there will be no max_virtqueue_pairs field if the
> >>>>>> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
> >>>>>> what spec said.
> >>>>>>
> >>>>>> Thanks  
> >>>>> I think that's a misunderstanding. This text was never intended to
> >>>>> imply that field offsets change beased on feature bits.
> >>>>> We had this pain with legacy and we never wanted to go back there.
> >>>>>
> >>>>> This merely implies that without VIRTIO_NET_F_MQ the field
> >>>>> should not be accessed. Exists in the sense "is accessible to driver".
> >>>>>
> >>>>> Let's just clarify that in the spec, job done.  
> >>>> Ok, agree. That will make things more eaiser.  
> >>> Yes, that makes much more sense.
> >>>
> >>> What about adding the following to the "Basic Facilities of a Virtio
> >>> Device/Device Configuration Space" section of the spec:
> >>>
> >>> "If an optional configuration field does not exist, the corresponding
> >>> space is still present, but reserved."  
> >>
> >> This became interesting after re-reading some of the qemu codes.
> >>
> >> E.g in virtio-net.c we had:
> >>
> >> *static VirtIOFeature feature_sizes[] = {
> >>       {.flags = 1ULL << VIRTIO_NET_F_MAC,
> >>        .end = endof(struct virtio_net_config, mac)},
> >>       {.flags = 1ULL << VIRTIO_NET_F_STATUS,
> >>        .end = endof(struct virtio_net_config, status)},
> >>       {.flags = 1ULL << VIRTIO_NET_F_MQ,
> >>        .end = endof(struct virtio_net_config, max_virtqueue_pairs)},
> >>       {.flags = 1ULL << VIRTIO_NET_F_MTU,
> >>        .end = endof(struct virtio_net_config, mtu)},
> >>       {.flags = 1ULL << VIRTIO_NET_F_SPEED_DUPLEX,
> >>        .end = endof(struct virtio_net_config, duplex)},
> >>       {.flags = (1ULL << VIRTIO_NET_F_RSS) | (1ULL <<
> >> VIRTIO_NET_F_HASH_REPORT),
> >>        .end = endof(struct virtio_net_config, supported_hash_types)},
> >>       {}
> >> };*
> >>
> >> *It has a implict dependency chain. E.g MTU doesn't presnet if
> >> DUPLEX/RSS is not offered ...
> >> *  
> > But I think it covers everything up to the relevant field, no? So MTU
> > is included if we have the feature bit, even if we don't have
> > DUPLEX/RSS.
> >
> > Given that a config space may be shorter (but must not collapse
> > non-existing fields), maybe a better wording would be:
> >
> > "If an optional configuration field does not exist, the corresponding
> > space will still be present if it is not at the end of the
> > configuration space (i.e., further configuration fields exist.)  
> 
> 
> This should work but I think we need to define the end of configuration 
> space first?

What about sidestepping this:

"...the corresponding space will still be present, unless no further
configuration fields exist."

?

> 
> > This
> > implies that a given field, if it exists, is always at the same offset
> > from the beginning of the configuration space."

Jason Wang March 5, 2021, 3:01 a.m. UTC | #63

On 2021/3/4 9:50 下午, Cornelia Huck wrote:
> On Thu, 4 Mar 2021 16:24:16 +0800
> Jason Wang <[e-mail]> wrote:
>
>> On 2021/3/3 4:29 下午, Cornelia Huck wrote:
>>> On Wed, 3 Mar 2021 12:01:01 +0800
>>> Jason Wang <[e-mail]> wrote:
>>>   
>>>> On 2021/3/2 8:08 下午, Cornelia Huck wrote:
>>>>> On Mon, 1 Mar 2021 11:51:08 +0800
>>>>> Jason Wang <[e-mail]> wrote:
>>>>>      
>>>>>> On 2021/3/1 5:25 上午, Michael S. Tsirkin wrote:
>>>>>>> On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:
>>>>>>>> On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:
>>>>>>>>> Confused. What is wrong with the above? It never reads the
>>>>>>>>> field unless the feature has been offered by device.
>>>>>>>> So the spec said:
>>>>>>>>
>>>>>>>> "
>>>>>>>>
>>>>>>>> The following driver-read-only field, max_virtqueue_pairs only exists if
>>>>>>>> VIRTIO_NET_F_MQ is set.
>>>>>>>>
>>>>>>>> "
>>>>>>>>
>>>>>>>> If I read this correctly, there will be no max_virtqueue_pairs field if the
>>>>>>>> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
>>>>>>>> what spec said.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>> I think that's a misunderstanding. This text was never intended to
>>>>>>> imply that field offsets change beased on feature bits.
>>>>>>> We had this pain with legacy and we never wanted to go back there.
>>>>>>>
>>>>>>> This merely implies that without VIRTIO_NET_F_MQ the field
>>>>>>> should not be accessed. Exists in the sense "is accessible to driver".
>>>>>>>
>>>>>>> Let's just clarify that in the spec, job done.
>>>>>> Ok, agree. That will make things more eaiser.
>>>>> Yes, that makes much more sense.
>>>>>
>>>>> What about adding the following to the "Basic Facilities of a Virtio
>>>>> Device/Device Configuration Space" section of the spec:
>>>>>
>>>>> "If an optional configuration field does not exist, the corresponding
>>>>> space is still present, but reserved."
>>>> This became interesting after re-reading some of the qemu codes.
>>>>
>>>> E.g in virtio-net.c we had:
>>>>
>>>> *static VirtIOFeature feature_sizes[] = {
>>>>        {.flags = 1ULL << VIRTIO_NET_F_MAC,
>>>>         .end = endof(struct virtio_net_config, mac)},
>>>>        {.flags = 1ULL << VIRTIO_NET_F_STATUS,
>>>>         .end = endof(struct virtio_net_config, status)},
>>>>        {.flags = 1ULL << VIRTIO_NET_F_MQ,
>>>>         .end = endof(struct virtio_net_config, max_virtqueue_pairs)},
>>>>        {.flags = 1ULL << VIRTIO_NET_F_MTU,
>>>>         .end = endof(struct virtio_net_config, mtu)},
>>>>        {.flags = 1ULL << VIRTIO_NET_F_SPEED_DUPLEX,
>>>>         .end = endof(struct virtio_net_config, duplex)},
>>>>        {.flags = (1ULL << VIRTIO_NET_F_RSS) | (1ULL <<
>>>> VIRTIO_NET_F_HASH_REPORT),
>>>>         .end = endof(struct virtio_net_config, supported_hash_types)},
>>>>        {}
>>>> };*
>>>>
>>>> *It has a implict dependency chain. E.g MTU doesn't presnet if
>>>> DUPLEX/RSS is not offered ...
>>>> *
>>> But I think it covers everything up to the relevant field, no? So MTU
>>> is included if we have the feature bit, even if we don't have
>>> DUPLEX/RSS.
>>>
>>> Given that a config space may be shorter (but must not collapse
>>> non-existing fields), maybe a better wording would be:
>>>
>>> "If an optional configuration field does not exist, the corresponding
>>> space will still be present if it is not at the end of the
>>> configuration space (i.e., further configuration fields exist.)
>>
>> This should work but I think we need to define the end of configuration
>> space first?
> What about sidestepping this:
>
> "...the corresponding space will still be present, unless no further
> configuration fields exist."
>
> ?


It might work. (I wonder maybe we can give some example in the spec).

Thanks


>
>>> This
>>> implies that a given field, if it exists, is always at the same offset
>>> from the beginning of the configuration space."

Si-Wei Liu Dec. 11, 2021, 1:44 a.m. UTC | #64

Sorry for reviving this ancient thread. I was kinda lost for the 
conclusion it ended up with. I have the following questions,

1. legacy guest support: from the past conversations it doesn't seem the 
support will be completely dropped from the table, is my understanding 
correct? Actually we're interested in supporting virtio v0.95 guest for 
x86, which is backed by the spec at 
https://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf. Though I'm not 
sure if there's request/need to support wilder legacy virtio versions 
earlier beyond.

2. suppose some form of legacy guest support needs to be there, how do 
we deal with the bogus assumption below in vdpa_get_config() in the 
short term? It looks one of the intuitive fix is to move the 
vdpa_set_features call out of vdpa_get_config() to vdpa_set_config().

         /*
          * Config accesses aren't supposed to trigger before features 
are set.
          * If it does happen we assume a legacy guest.
          */
         if (!vdev->features_valid)
                 vdpa_set_features(vdev, 0);
         ops->get_config(vdev, offset, buf, len);

I can post a patch to fix 2) if there's consensus already reached.

Thanks,
-Siwei

On 3/2/2021 2:53 AM, Jason Wang wrote:
>
> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>>> the kernel that it's a legacy guest. Ugh :(
>>>>> Well, although I think adding an ioctl is doable, may I know what 
>>>>> the use
>>>>> case there will be for kernel to leverage such info directly? Is 
>>>>> there a
>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>>> differentiation (legacy v.s. modern) needed?
>>>> BTW a good API could be
>>>>
>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>
>>>> we did it per vring but maybe that was a mistake ...
>>>
>>> Actually, I wonder whether it's good time to just not support legacy 
>>> driver
>>> for vDPA. Consider:
>>>
>>> 1) It's definition is no-normative
>>> 2) A lot of budren of codes
>>>
>>> So qemu can still present the legacy device since the config space 
>>> or other
>>> stuffs that is presented by vhost-vDPA is not expected to be 
>>> accessed by
>>> guest directly. Qemu can do the endian conversion when necessary in 
>>> this
>>> case?
>>>
>>> Thanks
>>>
>> Overall I would be fine with this approach but we need to avoid breaking
>> working userspace, qemu releases with vdpa support are out there and
>> seem to work for people. Any changes need to take that into account
>> and document compatibility concerns.
>
>
> Agree, let me check.
>
>
>>   I note that any hardware
>> implementation is already broken for legacy except on platforms with
>> strong ordering which might be helpful in reducing the scope.
>
>
> Yes.
>
> Thanks
>
>
>>
>>
>

Michael S. Tsirkin Dec. 12, 2021, 9:26 a.m. UTC | #65

On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> it ended up with. I have the following questions,
> 
> 1. legacy guest support: from the past conversations it doesn't seem the
> support will be completely dropped from the table, is my understanding
> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> which is backed by the spec at
> https://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf. Though I'm not sure
> if there's request/need to support wilder legacy virtio versions earlier
> beyond.

I personally feel it's less work to add in kernel than try to
work around it in userspace. Jason feels differently.
Maybe post the patches and this will prove to Jason it's not
too terrible?

> 2. suppose some form of legacy guest support needs to be there, how do we
> deal with the bogus assumption below in vdpa_get_config() in the short term?
> It looks one of the intuitive fix is to move the vdpa_set_features call out
> of vdpa_get_config() to vdpa_set_config().
> 
>         /*
>          * Config accesses aren't supposed to trigger before features are
> set.
>          * If it does happen we assume a legacy guest.
>          */
>         if (!vdev->features_valid)
>                 vdpa_set_features(vdev, 0);
>         ops->get_config(vdev, offset, buf, len);
> 
> I can post a patch to fix 2) if there's consensus already reached.
> 
> Thanks,
> -Siwei

I'm not sure how important it is to change that.
In any case it only affects transitional devices, right?
Legacy only should not care ...


> On 3/2/2021 2:53 AM, Jason Wang wrote:
> > 
> > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > > > > the kernel that it's a legacy guest. Ugh :(
> > > > > > Well, although I think adding an ioctl is doable, may I
> > > > > > know what the use
> > > > > > case there will be for kernel to leverage such info
> > > > > > directly? Is there a
> > > > > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > > > > differentiation (legacy v.s. modern) needed?
> > > > > BTW a good API could be
> > > > > 
> > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > 
> > > > > we did it per vring but maybe that was a mistake ...
> > > > 
> > > > Actually, I wonder whether it's good time to just not support
> > > > legacy driver
> > > > for vDPA. Consider:
> > > > 
> > > > 1) It's definition is no-normative
> > > > 2) A lot of budren of codes
> > > > 
> > > > So qemu can still present the legacy device since the config
> > > > space or other
> > > > stuffs that is presented by vhost-vDPA is not expected to be
> > > > accessed by
> > > > guest directly. Qemu can do the endian conversion when necessary
> > > > in this
> > > > case?
> > > > 
> > > > Thanks
> > > > 
> > > Overall I would be fine with this approach but we need to avoid breaking
> > > working userspace, qemu releases with vdpa support are out there and
> > > seem to work for people. Any changes need to take that into account
> > > and document compatibility concerns.
> > 
> > 
> > Agree, let me check.
> > 
> > 
> > >   I note that any hardware
> > > implementation is already broken for legacy except on platforms with
> > > strong ordering which might be helpful in reducing the scope.
> > 
> > 
> > Yes.
> > 
> > Thanks
> > 
> > 
> > > 
> > > 
> >

Jason Wang Dec. 13, 2021, 3:02 a.m. UTC | #66

On Sun, Dec 12, 2021 at 5:26 PM Michael S. Tsirkin <[e-mail]> wrote:
>
> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > it ended up with. I have the following questions,
> >
> > 1. legacy guest support: from the past conversations it doesn't seem the
> > support will be completely dropped from the table, is my understanding
> > correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > which is backed by the spec at
> > https://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf. Though I'm not sure
> > if there's request/need to support wilder legacy virtio versions earlier
> > beyond.
>
> I personally feel it's less work to add in kernel than try to
> work around it in userspace. Jason feels differently.
> Maybe post the patches and this will prove to Jason it's not
> too terrible?

That's one way, other than the config access before setting features,
we need to deal with other stuffs:

1) VIRTIO_F_ORDER_PLATFORM
2) there could be a parent device that only support 1.0 device

And a lot of other stuff summarized in spec 7.4 which seems not an
easy task. Various vDPA parent drivers were written under the
assumption that only modern devices are supported.

Thanks

>
> > 2. suppose some form of legacy guest support needs to be there, how do we
> > deal with the bogus assumption below in vdpa_get_config() in the short term?
> > It looks one of the intuitive fix is to move the vdpa_set_features call out
> > of vdpa_get_config() to vdpa_set_config().
> >
> >         /*
> >          * Config accesses aren't supposed to trigger before features are
> > set.
> >          * If it does happen we assume a legacy guest.
> >          */
> >         if (!vdev->features_valid)
> >                 vdpa_set_features(vdev, 0);
> >         ops->get_config(vdev, offset, buf, len);
> >
> > I can post a patch to fix 2) if there's consensus already reached.
> >
> > Thanks,
> > -Siwei
>
> I'm not sure how important it is to change that.
> In any case it only affects transitional devices, right?
> Legacy only should not care ...
>
>
> > On 3/2/2021 2:53 AM, Jason Wang wrote:
> > >
> > > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > > > > > the kernel that it's a legacy guest. Ugh :(
> > > > > > > Well, although I think adding an ioctl is doable, may I
> > > > > > > know what the use
> > > > > > > case there will be for kernel to leverage such info
> > > > > > > directly? Is there a
> > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > > > > > differentiation (legacy v.s. modern) needed?
> > > > > > BTW a good API could be
> > > > > >
> > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > >
> > > > > > we did it per vring but maybe that was a mistake ...
> > > > >
> > > > > Actually, I wonder whether it's good time to just not support
> > > > > legacy driver
> > > > > for vDPA. Consider:
> > > > >
> > > > > 1) It's definition is no-normative
> > > > > 2) A lot of budren of codes
> > > > >
> > > > > So qemu can still present the legacy device since the config
> > > > > space or other
> > > > > stuffs that is presented by vhost-vDPA is not expected to be
> > > > > accessed by
> > > > > guest directly. Qemu can do the endian conversion when necessary
> > > > > in this
> > > > > case?
> > > > >
> > > > > Thanks
> > > > >
> > > > Overall I would be fine with this approach but we need to avoid breaking
> > > > working userspace, qemu releases with vdpa support are out there and
> > > > seem to work for people. Any changes need to take that into account
> > > > and document compatibility concerns.
> > >
> > >
> > > Agree, let me check.
> > >
> > >
> > > >   I note that any hardware
> > > > implementation is already broken for legacy except on platforms with
> > > > strong ordering which might be helpful in reducing the scope.
> > >
> > >
> > > Yes.
> > >
> > > Thanks
> > >
> > >
> > > >
> > > >
> > >
>

Michael S. Tsirkin Dec. 13, 2021, 8:06 a.m. UTC | #67

On Mon, Dec 13, 2021 at 11:02:39AM +0800, Jason Wang wrote:
> On Sun, Dec 12, 2021 at 5:26 PM Michael S. Tsirkin <[e-mail]> wrote:
> >
> > On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > > Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > > it ended up with. I have the following questions,
> > >
> > > 1. legacy guest support: from the past conversations it doesn't seem the
> > > support will be completely dropped from the table, is my understanding
> > > correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > > which is backed by the spec at
> > > https://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf. Though I'm not sure
> > > if there's request/need to support wilder legacy virtio versions earlier
> > > beyond.
> >
> > I personally feel it's less work to add in kernel than try to
> > work around it in userspace. Jason feels differently.
> > Maybe post the patches and this will prove to Jason it's not
> > too terrible?
> 
> That's one way, other than the config access before setting features,
> we need to deal with other stuffs:
> 
> 1) VIRTIO_F_ORDER_PLATFORM
> 2) there could be a parent device that only support 1.0 device
> 
> And a lot of other stuff summarized in spec 7.4 which seems not an
> easy task. Various vDPA parent drivers were written under the
> assumption that only modern devices are supported.
> 
> Thanks

Limiting things to x86 will likely address most issues though, won't it?

> >
> > > 2. suppose some form of legacy guest support needs to be there, how do we
> > > deal with the bogus assumption below in vdpa_get_config() in the short term?
> > > It looks one of the intuitive fix is to move the vdpa_set_features call out
> > > of vdpa_get_config() to vdpa_set_config().
> > >
> > >         /*
> > >          * Config accesses aren't supposed to trigger before features are
> > > set.
> > >          * If it does happen we assume a legacy guest.
> > >          */
> > >         if (!vdev->features_valid)
> > >                 vdpa_set_features(vdev, 0);
> > >         ops->get_config(vdev, offset, buf, len);
> > >
> > > I can post a patch to fix 2) if there's consensus already reached.
> > >
> > > Thanks,
> > > -Siwei
> >
> > I'm not sure how important it is to change that.
> > In any case it only affects transitional devices, right?
> > Legacy only should not care ...
> >
> >
> > > On 3/2/2021 2:53 AM, Jason Wang wrote:
> > > >
> > > > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > > > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > > > > > > the kernel that it's a legacy guest. Ugh :(
> > > > > > > > Well, although I think adding an ioctl is doable, may I
> > > > > > > > know what the use
> > > > > > > > case there will be for kernel to leverage such info
> > > > > > > > directly? Is there a
> > > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > > > > > > differentiation (legacy v.s. modern) needed?
> > > > > > > BTW a good API could be
> > > > > > >
> > > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > >
> > > > > > > we did it per vring but maybe that was a mistake ...
> > > > > >
> > > > > > Actually, I wonder whether it's good time to just not support
> > > > > > legacy driver
> > > > > > for vDPA. Consider:
> > > > > >
> > > > > > 1) It's definition is no-normative
> > > > > > 2) A lot of budren of codes
> > > > > >
> > > > > > So qemu can still present the legacy device since the config
> > > > > > space or other
> > > > > > stuffs that is presented by vhost-vDPA is not expected to be
> > > > > > accessed by
> > > > > > guest directly. Qemu can do the endian conversion when necessary
> > > > > > in this
> > > > > > case?
> > > > > >
> > > > > > Thanks
> > > > > >
> > > > > Overall I would be fine with this approach but we need to avoid breaking
> > > > > working userspace, qemu releases with vdpa support are out there and
> > > > > seem to work for people. Any changes need to take that into account
> > > > > and document compatibility concerns.
> > > >
> > > >
> > > > Agree, let me check.
> > > >
> > > >
> > > > >   I note that any hardware
> > > > > implementation is already broken for legacy except on platforms with
> > > > > strong ordering which might be helpful in reducing the scope.
> > > >
> > > >
> > > > Yes.
> > > >
> > > > Thanks
> > > >
> > > >
> > > > >
> > > > >
> > > >
> >

Jason Wang Dec. 13, 2021, 8:57 a.m. UTC | #68

On Mon, Dec 13, 2021 at 4:07 PM Michael S. Tsirkin <[e-mail]> wrote:
>
> On Mon, Dec 13, 2021 at 11:02:39AM +0800, Jason Wang wrote:
> > On Sun, Dec 12, 2021 at 5:26 PM Michael S. Tsirkin <[e-mail]> wrote:
> > >
> > > On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > > > Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > > > it ended up with. I have the following questions,
> > > >
> > > > 1. legacy guest support: from the past conversations it doesn't seem the
> > > > support will be completely dropped from the table, is my understanding
> > > > correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > > > which is backed by the spec at
> > > > https://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf. Though I'm not sure
> > > > if there's request/need to support wilder legacy virtio versions earlier
> > > > beyond.
> > >
> > > I personally feel it's less work to add in kernel than try to
> > > work around it in userspace. Jason feels differently.
> > > Maybe post the patches and this will prove to Jason it's not
> > > too terrible?
> >
> > That's one way, other than the config access before setting features,
> > we need to deal with other stuffs:
> >
> > 1) VIRTIO_F_ORDER_PLATFORM
> > 2) there could be a parent device that only support 1.0 device
> >
> > And a lot of other stuff summarized in spec 7.4 which seems not an
> > easy task. Various vDPA parent drivers were written under the
> > assumption that only modern devices are supported.
> >
> > Thanks
>
> Limiting things to x86 will likely address most issues though, won't it?

For the ordering, yes. But it means we need to introduce a config
option for legacy logic?

And we need to deal with, as you said in another thread, kick before DRIVER_OK:

E.g we had thing like this:

        if ((status & VIRTIO_CONFIG_S_DRIVER_OK) &&
            !(status_old & VIRTIO_CONFIG_S_DRIVER_OK)) {
                ret = ifcvf_request_irq(adapter);
                if (ret) {

Similar issues could be found in other parents.

We also need to consider whether we should encourage the vendor to
implement the logic.

I think we can try and see how hard it is.

Thanks

>
> > >
> > > > 2. suppose some form of legacy guest support needs to be there, how do we
> > > > deal with the bogus assumption below in vdpa_get_config() in the short term?
> > > > It looks one of the intuitive fix is to move the vdpa_set_features call out
> > > > of vdpa_get_config() to vdpa_set_config().
> > > >
> > > >         /*
> > > >          * Config accesses aren't supposed to trigger before features are
> > > > set.
> > > >          * If it does happen we assume a legacy guest.
> > > >          */
> > > >         if (!vdev->features_valid)
> > > >                 vdpa_set_features(vdev, 0);
> > > >         ops->get_config(vdev, offset, buf, len);
> > > >
> > > > I can post a patch to fix 2) if there's consensus already reached.
> > > >
> > > > Thanks,
> > > > -Siwei
> > >
> > > I'm not sure how important it is to change that.
> > > In any case it only affects transitional devices, right?
> > > Legacy only should not care ...
> > >
> > >
> > > > On 3/2/2021 2:53 AM, Jason Wang wrote:
> > > > >
> > > > > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > > > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > > > > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > > > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > > > > > > > the kernel that it's a legacy guest. Ugh :(
> > > > > > > > > Well, although I think adding an ioctl is doable, may I
> > > > > > > > > know what the use
> > > > > > > > > case there will be for kernel to leverage such info
> > > > > > > > > directly? Is there a
> > > > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > > > > > > > differentiation (legacy v.s. modern) needed?
> > > > > > > > BTW a good API could be
> > > > > > > >
> > > > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > >
> > > > > > > > we did it per vring but maybe that was a mistake ...
> > > > > > >
> > > > > > > Actually, I wonder whether it's good time to just not support
> > > > > > > legacy driver
> > > > > > > for vDPA. Consider:
> > > > > > >
> > > > > > > 1) It's definition is no-normative
> > > > > > > 2) A lot of budren of codes
> > > > > > >
> > > > > > > So qemu can still present the legacy device since the config
> > > > > > > space or other
> > > > > > > stuffs that is presented by vhost-vDPA is not expected to be
> > > > > > > accessed by
> > > > > > > guest directly. Qemu can do the endian conversion when necessary
> > > > > > > in this
> > > > > > > case?
> > > > > > >
> > > > > > > Thanks
> > > > > > >
> > > > > > Overall I would be fine with this approach but we need to avoid breaking
> > > > > > working userspace, qemu releases with vdpa support are out there and
> > > > > > seem to work for people. Any changes need to take that into account
> > > > > > and document compatibility concerns.
> > > > >
> > > > >
> > > > > Agree, let me check.
> > > > >
> > > > >
> > > > > >   I note that any hardware
> > > > > > implementation is already broken for legacy except on platforms with
> > > > > > strong ordering which might be helpful in reducing the scope.
> > > > >
> > > > >
> > > > > Yes.
> > > > >
> > > > > Thanks
> > > > >
> > > > >
> > > > > >
> > > > > >
> > > > >
> > >
>

Michael S. Tsirkin Dec. 13, 2021, 10:42 a.m. UTC | #69

On Mon, Dec 13, 2021 at 04:57:38PM +0800, Jason Wang wrote:
> On Mon, Dec 13, 2021 at 4:07 PM Michael S. Tsirkin <[e-mail]> wrote:
> >
> > On Mon, Dec 13, 2021 at 11:02:39AM +0800, Jason Wang wrote:
> > > On Sun, Dec 12, 2021 at 5:26 PM Michael S. Tsirkin <[e-mail]> wrote:
> > > >
> > > > On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > > > > Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > > > > it ended up with. I have the following questions,
> > > > >
> > > > > 1. legacy guest support: from the past conversations it doesn't seem the
> > > > > support will be completely dropped from the table, is my understanding
> > > > > correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > > > > which is backed by the spec at
> > > > > https://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf. Though I'm not sure
> > > > > if there's request/need to support wilder legacy virtio versions earlier
> > > > > beyond.
> > > >
> > > > I personally feel it's less work to add in kernel than try to
> > > > work around it in userspace. Jason feels differently.
> > > > Maybe post the patches and this will prove to Jason it's not
> > > > too terrible?
> > >
> > > That's one way, other than the config access before setting features,
> > > we need to deal with other stuffs:
> > >
> > > 1) VIRTIO_F_ORDER_PLATFORM
> > > 2) there could be a parent device that only support 1.0 device
> > >
> > > And a lot of other stuff summarized in spec 7.4 which seems not an
> > > easy task. Various vDPA parent drivers were written under the
> > > assumption that only modern devices are supported.
> > >
> > > Thanks
> >
> > Limiting things to x86 will likely address most issues though, won't it?
> 
> For the ordering, yes. But it means we need to introduce a config
> option for legacy logic?
> And we need to deal with, as you said in another thread, kick before DRIVER_OK:
> 
> E.g we had thing like this:
> 
>         if ((status & VIRTIO_CONFIG_S_DRIVER_OK) &&
>             !(status_old & VIRTIO_CONFIG_S_DRIVER_OK)) {
>                 ret = ifcvf_request_irq(adapter);
>                 if (ret) {
> 
> Similar issues could be found in other parents.

The driver ok thing is mostly an issue for block where it
expects to access the disk directly during probe.

> We also need to consider whether we should encourage the vendor to
> implement the logic.
> 
> I think we can try and see how hard it is.
> 
> Thanks

Right. My point exactly.

> >
> > > >
> > > > > 2. suppose some form of legacy guest support needs to be there, how do we
> > > > > deal with the bogus assumption below in vdpa_get_config() in the short term?
> > > > > It looks one of the intuitive fix is to move the vdpa_set_features call out
> > > > > of vdpa_get_config() to vdpa_set_config().
> > > > >
> > > > >         /*
> > > > >          * Config accesses aren't supposed to trigger before features are
> > > > > set.
> > > > >          * If it does happen we assume a legacy guest.
> > > > >          */
> > > > >         if (!vdev->features_valid)
> > > > >                 vdpa_set_features(vdev, 0);
> > > > >         ops->get_config(vdev, offset, buf, len);
> > > > >
> > > > > I can post a patch to fix 2) if there's consensus already reached.
> > > > >
> > > > > Thanks,
> > > > > -Siwei
> > > >
> > > > I'm not sure how important it is to change that.
> > > > In any case it only affects transitional devices, right?
> > > > Legacy only should not care ...
> > > >
> > > >
> > > > > On 3/2/2021 2:53 AM, Jason Wang wrote:
> > > > > >
> > > > > > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > > > > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > > > > > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > > > > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > > > > > > > > the kernel that it's a legacy guest. Ugh :(
> > > > > > > > > > Well, although I think adding an ioctl is doable, may I
> > > > > > > > > > know what the use
> > > > > > > > > > case there will be for kernel to leverage such info
> > > > > > > > > > directly? Is there a
> > > > > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > > > > > > > > differentiation (legacy v.s. modern) needed?
> > > > > > > > > BTW a good API could be
> > > > > > > > >
> > > > > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > > >
> > > > > > > > > we did it per vring but maybe that was a mistake ...
> > > > > > > >
> > > > > > > > Actually, I wonder whether it's good time to just not support
> > > > > > > > legacy driver
> > > > > > > > for vDPA. Consider:
> > > > > > > >
> > > > > > > > 1) It's definition is no-normative
> > > > > > > > 2) A lot of budren of codes
> > > > > > > >
> > > > > > > > So qemu can still present the legacy device since the config
> > > > > > > > space or other
> > > > > > > > stuffs that is presented by vhost-vDPA is not expected to be
> > > > > > > > accessed by
> > > > > > > > guest directly. Qemu can do the endian conversion when necessary
> > > > > > > > in this
> > > > > > > > case?
> > > > > > > >
> > > > > > > > Thanks
> > > > > > > >
> > > > > > > Overall I would be fine with this approach but we need to avoid breaking
> > > > > > > working userspace, qemu releases with vdpa support are out there and
> > > > > > > seem to work for people. Any changes need to take that into account
> > > > > > > and document compatibility concerns.
> > > > > >
> > > > > >
> > > > > > Agree, let me check.
> > > > > >
> > > > > >
> > > > > > >   I note that any hardware
> > > > > > > implementation is already broken for legacy except on platforms with
> > > > > > > strong ordering which might be helpful in reducing the scope.
> > > > > >
> > > > > >
> > > > > > Yes.
> > > > > >
> > > > > > Thanks
> > > > > >
> > > > > >
> > > > > > >
> > > > > > >
> > > > > >
> > > >
> >

Si-Wei Liu Dec. 14, 2021, 1:13 a.m. UTC | #70

On 12/12/2021 7:02 PM, Jason Wang wrote:
> On Sun, Dec 12, 2021 at 5:26 PM Michael S. Tsirkin <[e-mail]> wrote:
>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
>>> it ended up with. I have the following questions,
>>>
>>> 1. legacy guest support: from the past conversations it doesn't seem the
>>> support will be completely dropped from the table, is my understanding
>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
>>> which is backed by the spec at
>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!f64RqPFbYWWTGBgfWLzjlpJR_89WgX9KQTTz2vd-9UvMufMzqEbsajs8dxSfg0G8$ . Though I'm not sure
>>> if there's request/need to support wilder legacy virtio versions earlier
>>> beyond.
>> I personally feel it's less work to add in kernel than try to
>> work around it in userspace. Jason feels differently.
>> Maybe post the patches and this will prove to Jason it's not
>> too terrible?
> That's one way, other than the config access before setting features,
> we need to deal with other stuffs:
>
> 1) VIRTIO_F_ORDER_PLATFORM
> 2) there could be a parent device that only support 1.0 device
We do want to involve vendor's support for a legacy (or transitional) 
device datapath. Otherwise it'd be too difficult to emulate/translate in 
software/QEMU. The above two might not be an issue if the vendor claims 
0.95 support in virtqueue and ring layout, plus limiting to x86 support 
(LE with weak ordering) seems to simplify a lot of these requirements. I 
don't think emulating a legacy device model on top of a 1.0 vdpa parent 
for the dataplane would be a good idea, either.

>
> And a lot of other stuff summarized in spec 7.4 which seems not an
> easy task. Various vDPA parent drivers were written under the
> assumption that only modern devices are supported.
If some of these vDPA vendors do provide the 0.95 support, especially on 
the datapath and ring layout that well satisfies a transitional device 
model defined in section 7.4, I guess we can scope the initial support 
to these vendor drivers and x86 only. Let me know if I miss something else.

Thanks,
-Siwei


>
> Thanks
>
>>> 2. suppose some form of legacy guest support needs to be there, how do we
>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
>>> of vdpa_get_config() to vdpa_set_config().
>>>
>>>          /*
>>>           * Config accesses aren't supposed to trigger before features are
>>> set.
>>>           * If it does happen we assume a legacy guest.
>>>           */
>>>          if (!vdev->features_valid)
>>>                  vdpa_set_features(vdev, 0);
>>>          ops->get_config(vdev, offset, buf, len);
>>>
>>> I can post a patch to fix 2) if there's consensus already reached.
>>>
>>> Thanks,
>>> -Siwei
>> I'm not sure how important it is to change that.
>> In any case it only affects transitional devices, right?
>> Legacy only should not care ...
>>
>>
>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
>>>>>>>> Well, although I think adding an ioctl is doable, may I
>>>>>>>> know what the use
>>>>>>>> case there will be for kernel to leverage such info
>>>>>>>> directly? Is there a
>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>>>>>> differentiation (legacy v.s. modern) needed?
>>>>>>> BTW a good API could be
>>>>>>>
>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>
>>>>>>> we did it per vring but maybe that was a mistake ...
>>>>>> Actually, I wonder whether it's good time to just not support
>>>>>> legacy driver
>>>>>> for vDPA. Consider:
>>>>>>
>>>>>> 1) It's definition is no-normative
>>>>>> 2) A lot of budren of codes
>>>>>>
>>>>>> So qemu can still present the legacy device since the config
>>>>>> space or other
>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
>>>>>> accessed by
>>>>>> guest directly. Qemu can do the endian conversion when necessary
>>>>>> in this
>>>>>> case?
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>> Overall I would be fine with this approach but we need to avoid breaking
>>>>> working userspace, qemu releases with vdpa support are out there and
>>>>> seem to work for people. Any changes need to take that into account
>>>>> and document compatibility concerns.
>>>>
>>>> Agree, let me check.
>>>>
>>>>
>>>>>    I note that any hardware
>>>>> implementation is already broken for legacy except on platforms with
>>>>> strong ordering which might be helpful in reducing the scope.
>>>>
>>>> Yes.
>>>>
>>>> Thanks
>>>>
>>>>
>>>>>

Si-Wei Liu Dec. 14, 2021, 1:59 a.m. UTC | #71

On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
>> it ended up with. I have the following questions,
>>
>> 1. legacy guest support: from the past conversations it doesn't seem the
>> support will be completely dropped from the table, is my understanding
>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
>> which is backed by the spec at
>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
>> if there's request/need to support wilder legacy virtio versions earlier
>> beyond.
> I personally feel it's less work to add in kernel than try to
> work around it in userspace. Jason feels differently.
> Maybe post the patches and this will prove to Jason it's not
> too terrible?
I suppose if the vdpa vendor does support 0.95 in the datapath and ring 
layout level and is limited to x86 only, there should be easy way out. I 
checked with Eli and other Mellanox/NVDIA folks for hardware/firmware 
level 0.95 support, it seems all the ingredient had been there already 
dated back to the DPDK days. The only major thing limiting is in the 
vDPA software that the current vdpa core has the assumption around 
VIRTIO_F_ACCESS_PLATFORM for a few DMA setup ops, which is virtio 1.0 only.

>
>> 2. suppose some form of legacy guest support needs to be there, how do we
>> deal with the bogus assumption below in vdpa_get_config() in the short term?
>> It looks one of the intuitive fix is to move the vdpa_set_features call out
>> of vdpa_get_config() to vdpa_set_config().
>>
>>          /*
>>           * Config accesses aren't supposed to trigger before features are
>> set.
>>           * If it does happen we assume a legacy guest.
>>           */
>>          if (!vdev->features_valid)
>>                  vdpa_set_features(vdev, 0);
>>          ops->get_config(vdev, offset, buf, len);
>>
>> I can post a patch to fix 2) if there's consensus already reached.
>>
>> Thanks,
>> -Siwei
> I'm not sure how important it is to change that.
> In any case it only affects transitional devices, right?
> Legacy only should not care ...
Yes I'd like to distinguish legacy driver (suppose it is 0.95) against 
the modern one in a transitional device model rather than being legacy 
only. That way a v0.95 and v1.0 supporting vdpa parent can support both 
types of guests without having to reconfigure. Or are you suggesting 
limit to legacy only at the time of vdpa creation would simplify the 
implementation a lot?

Thanks,
-Siwei

>
>> On 3/2/2021 2:53 AM, Jason Wang wrote:
>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>>>>> the kernel that it's a legacy guest. Ugh :(
>>>>>>> Well, although I think adding an ioctl is doable, may I
>>>>>>> know what the use
>>>>>>> case there will be for kernel to leverage such info
>>>>>>> directly? Is there a
>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>>>>> differentiation (legacy v.s. modern) needed?
>>>>>> BTW a good API could be
>>>>>>
>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>
>>>>>> we did it per vring but maybe that was a mistake ...
>>>>> Actually, I wonder whether it's good time to just not support
>>>>> legacy driver
>>>>> for vDPA. Consider:
>>>>>
>>>>> 1) It's definition is no-normative
>>>>> 2) A lot of budren of codes
>>>>>
>>>>> So qemu can still present the legacy device since the config
>>>>> space or other
>>>>> stuffs that is presented by vhost-vDPA is not expected to be
>>>>> accessed by
>>>>> guest directly. Qemu can do the endian conversion when necessary
>>>>> in this
>>>>> case?
>>>>>
>>>>> Thanks
>>>>>
>>>> Overall I would be fine with this approach but we need to avoid breaking
>>>> working userspace, qemu releases with vdpa support are out there and
>>>> seem to work for people. Any changes need to take that into account
>>>> and document compatibility concerns.
>>>
>>> Agree, let me check.
>>>
>>>
>>>>    I note that any hardware
>>>> implementation is already broken for legacy except on platforms with
>>>> strong ordering which might be helpful in reducing the scope.
>>>
>>> Yes.
>>>
>>> Thanks
>>>
>>>
>>>>

Jason Wang Dec. 14, 2021, 3:01 a.m. UTC | #72

On Tue, Dec 14, 2021 at 10:00 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> > On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> >> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> >> it ended up with. I have the following questions,
> >>
> >> 1. legacy guest support: from the past conversations it doesn't seem the
> >> support will be completely dropped from the table, is my understanding
> >> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> >> which is backed by the spec at
> >> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> >> if there's request/need to support wilder legacy virtio versions earlier
> >> beyond.
> > I personally feel it's less work to add in kernel than try to
> > work around it in userspace. Jason feels differently.
> > Maybe post the patches and this will prove to Jason it's not
> > too terrible?
> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> layout level and is limited to x86 only, there should be easy way out.

Note that thought I try to mandate 1.0 device when writing the codes
but the core vdpa doesn't mandate it, and we've already had one parent
which is based on the 0.95 spec which is the eni_vdpa:

1) it depends on X86 (so no endian and ordering issues)
2) it has various subtle things like it can't work well without
mrg_rxbuf features negotiated since the device assumes a fixed vnet
header length.
3) it can only be used by legacy drivers in the guest (no VERSION_1
since the device mandates a 4096 alignment which doesn't comply with
1.0)

So it's a proof of 0.95 parent support in the vDPA core.

And we had a modern only parent, that is the vp_vdpa parent (though
it's not hard to add legacy support).

So for all the other vendors, assuming it has full support for
transitional devices for x86. As discussed, we need to handle:

1) config access before features
2) kick before driver_ok

Anything else? If not, it looks easier to do them in the userspace.
The only advantages for doing it in the kernel is to make it work for
virtio-vdpa. But virito-vdpa doesn't need transitional devices.

> I
> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware
> level 0.95 support, it seems all the ingredient had been there already
> dated back to the DPDK days. The only major thing limiting is in the
> vDPA software that the current vdpa core has the assumption around
> VIRTIO_F_ACCESS_PLATFORM for a few DMA setup ops, which is virtio 1.0 only.

The code doesn't have such an assumption or anything I missed? Or you
meant the vhost-vdpa that tries to talk with the IOMMU layer directly,
it should be ok since host IOMMU is hidden from guest anyway.

>
> >
> >> 2. suppose some form of legacy guest support needs to be there, how do we
> >> deal with the bogus assumption below in vdpa_get_config() in the short term?
> >> It looks one of the intuitive fix is to move the vdpa_set_features call out
> >> of vdpa_get_config() to vdpa_set_config().
> >>
> >>          /*
> >>           * Config accesses aren't supposed to trigger before features are
> >> set.
> >>           * If it does happen we assume a legacy guest.
> >>           */
> >>          if (!vdev->features_valid)
> >>                  vdpa_set_features(vdev, 0);
> >>          ops->get_config(vdev, offset, buf, len);
> >>
> >> I can post a patch to fix 2) if there's consensus already reached.
> >>
> >> Thanks,
> >> -Siwei
> > I'm not sure how important it is to change that.
> > In any case it only affects transitional devices, right?
> > Legacy only should not care ...
> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against
> the modern one in a transitional device model rather than being legacy
> only. That way a v0.95 and v1.0 supporting vdpa parent can support both
> types of guests without having to reconfigure.

I think this is what a transitional device is expected to work.

Thanks

> Or are you suggesting
> limit to legacy only at the time of vdpa creation would simplify the
> implementation a lot?
>
> Thanks,
> -Siwei
>
> >
> >> On 3/2/2021 2:53 AM, Jason Wang wrote:
> >>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> >>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> >>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> >>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> >>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
> >>>>>>>> the kernel that it's a legacy guest. Ugh :(
> >>>>>>> Well, although I think adding an ioctl is doable, may I
> >>>>>>> know what the use
> >>>>>>> case there will be for kernel to leverage such info
> >>>>>>> directly? Is there a
> >>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
> >>>>>>> differentiation (legacy v.s. modern) needed?
> >>>>>> BTW a good API could be
> >>>>>>
> >>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>
> >>>>>> we did it per vring but maybe that was a mistake ...
> >>>>> Actually, I wonder whether it's good time to just not support
> >>>>> legacy driver
> >>>>> for vDPA. Consider:
> >>>>>
> >>>>> 1) It's definition is no-normative
> >>>>> 2) A lot of budren of codes
> >>>>>
> >>>>> So qemu can still present the legacy device since the config
> >>>>> space or other
> >>>>> stuffs that is presented by vhost-vDPA is not expected to be
> >>>>> accessed by
> >>>>> guest directly. Qemu can do the endian conversion when necessary
> >>>>> in this
> >>>>> case?
> >>>>>
> >>>>> Thanks
> >>>>>
> >>>> Overall I would be fine with this approach but we need to avoid breaking
> >>>> working userspace, qemu releases with vdpa support are out there and
> >>>> seem to work for people. Any changes need to take that into account
> >>>> and document compatibility concerns.
> >>>
> >>> Agree, let me check.
> >>>
> >>>
> >>>>    I note that any hardware
> >>>> implementation is already broken for legacy except on platforms with
> >>>> strong ordering which might be helpful in reducing the scope.
> >>>
> >>> Yes.
> >>>
> >>> Thanks
> >>>
> >>>
> >>>>
>

Michael S. Tsirkin Dec. 14, 2021, 5:06 a.m. UTC | #73

On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> 
> 
> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> > On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > > Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > > it ended up with. I have the following questions,
> > > 
> > > 1. legacy guest support: from the past conversations it doesn't seem the
> > > support will be completely dropped from the table, is my understanding
> > > correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > > which is backed by the spec at
> > > https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> > > if there's request/need to support wilder legacy virtio versions earlier
> > > beyond.
> > I personally feel it's less work to add in kernel than try to
> > work around it in userspace. Jason feels differently.
> > Maybe post the patches and this will prove to Jason it's not
> > too terrible?
> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> layout level and is limited to x86 only, there should be easy way out.

Note a subtle difference: what matters is that guest, not host is x86.
Matters for emulators which might reorder memory accesses.
I guess this enforcement belongs in QEMU then?

> I
> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> 0.95 support, it seems all the ingredient had been there already dated back
> to the DPDK days. The only major thing limiting is in the vDPA software that
> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> a few DMA setup ops, which is virtio 1.0 only.
> 
> > 
> > > 2. suppose some form of legacy guest support needs to be there, how do we
> > > deal with the bogus assumption below in vdpa_get_config() in the short term?
> > > It looks one of the intuitive fix is to move the vdpa_set_features call out
> > > of vdpa_get_config() to vdpa_set_config().
> > > 
> > >          /*
> > >           * Config accesses aren't supposed to trigger before features are
> > > set.
> > >           * If it does happen we assume a legacy guest.
> > >           */
> > >          if (!vdev->features_valid)
> > >                  vdpa_set_features(vdev, 0);
> > >          ops->get_config(vdev, offset, buf, len);
> > > 
> > > I can post a patch to fix 2) if there's consensus already reached.
> > > 
> > > Thanks,
> > > -Siwei
> > I'm not sure how important it is to change that.
> > In any case it only affects transitional devices, right?
> > Legacy only should not care ...
> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> modern one in a transitional device model rather than being legacy only.
> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> guests without having to reconfigure. Or are you suggesting limit to legacy
> only at the time of vdpa creation would simplify the implementation a lot?
> 
> Thanks,
> -Siwei


I don't know for sure. Take a look at the work Halil was doing
to try and support transitional devices with BE guests.


> > 
> > > On 3/2/2021 2:53 AM, Jason Wang wrote:
> > > > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > > > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > > > > > > the kernel that it's a legacy guest. Ugh :(
> > > > > > > > Well, although I think adding an ioctl is doable, may I
> > > > > > > > know what the use
> > > > > > > > case there will be for kernel to leverage such info
> > > > > > > > directly? Is there a
> > > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > > > > > > differentiation (legacy v.s. modern) needed?
> > > > > > > BTW a good API could be
> > > > > > > 
> > > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > 
> > > > > > > we did it per vring but maybe that was a mistake ...
> > > > > > Actually, I wonder whether it's good time to just not support
> > > > > > legacy driver
> > > > > > for vDPA. Consider:
> > > > > > 
> > > > > > 1) It's definition is no-normative
> > > > > > 2) A lot of budren of codes
> > > > > > 
> > > > > > So qemu can still present the legacy device since the config
> > > > > > space or other
> > > > > > stuffs that is presented by vhost-vDPA is not expected to be
> > > > > > accessed by
> > > > > > guest directly. Qemu can do the endian conversion when necessary
> > > > > > in this
> > > > > > case?
> > > > > > 
> > > > > > Thanks
> > > > > > 
> > > > > Overall I would be fine with this approach but we need to avoid breaking
> > > > > working userspace, qemu releases with vdpa support are out there and
> > > > > seem to work for people. Any changes need to take that into account
> > > > > and document compatibility concerns.
> > > > 
> > > > Agree, let me check.
> > > > 
> > > > 
> > > > >    I note that any hardware
> > > > > implementation is already broken for legacy except on platforms with
> > > > > strong ordering which might be helpful in reducing the scope.
> > > > 
> > > > Yes.
> > > > 
> > > > Thanks
> > > > 
> > > > 
> > > > >

Si-Wei Liu Dec. 15, 2021, 1:05 a.m. UTC | #74

On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
>>
>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
>>>> it ended up with. I have the following questions,
>>>>
>>>> 1. legacy guest support: from the past conversations it doesn't seem the
>>>> support will be completely dropped from the table, is my understanding
>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
>>>> which is backed by the spec at
>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
>>>> if there's request/need to support wilder legacy virtio versions earlier
>>>> beyond.
>>> I personally feel it's less work to add in kernel than try to
>>> work around it in userspace. Jason feels differently.
>>> Maybe post the patches and this will prove to Jason it's not
>>> too terrible?
>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
>> layout level and is limited to x86 only, there should be easy way out.
> Note a subtle difference: what matters is that guest, not host is x86.
> Matters for emulators which might reorder memory accesses.
> I guess this enforcement belongs in QEMU then?
Right, I mean to get started, the initial guest driver support and the 
corresponding QEMU support for transitional vdpa backend can be limited 
to x86 guest/host only. Since the config space is emulated in QEMU, I 
suppose it's not hard to enforce in QEMU. QEMU can drive GET_LEGACY, 
GET_ENDIAN et al ioctls in advance to get the capability from the 
individual vendor driver. For that, we need another negotiation protocol 
similar to vhost_user's protocol_features between the vdpa kernel and 
QEMU, way before the guest driver is ever probed and its feature 
negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call 
from the device, but we can assume weak ordering for legacy at this 
point (x86 only)?

>
>> I
>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
>> 0.95 support, it seems all the ingredient had been there already dated back
>> to the DPDK days. The only major thing limiting is in the vDPA software that
>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
>> a few DMA setup ops, which is virtio 1.0 only.
>>
>>>> 2. suppose some form of legacy guest support needs to be there, how do we
>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
>>>> of vdpa_get_config() to vdpa_set_config().
>>>>
>>>>           /*
>>>>            * Config accesses aren't supposed to trigger before features are
>>>> set.
>>>>            * If it does happen we assume a legacy guest.
>>>>            */
>>>>           if (!vdev->features_valid)
>>>>                   vdpa_set_features(vdev, 0);
>>>>           ops->get_config(vdev, offset, buf, len);
>>>>
>>>> I can post a patch to fix 2) if there's consensus already reached.
>>>>
>>>> Thanks,
>>>> -Siwei
>>> I'm not sure how important it is to change that.
>>> In any case it only affects transitional devices, right?
>>> Legacy only should not care ...
>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
>> modern one in a transitional device model rather than being legacy only.
>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
>> guests without having to reconfigure. Or are you suggesting limit to legacy
>> only at the time of vdpa creation would simplify the implementation a lot?
>>
>> Thanks,
>> -Siwei
>
> I don't know for sure. Take a look at the work Halil was doing
> to try and support transitional devices with BE guests.
Hmmm, we can have those endianness ioctls defined but the initial QEMU 
implementation can be started to support x86 guest/host with little 
endian and weak memory ordering first. The real trick is to detect 
legacy guest - I am not sure if it's feasible to shift all the legacy 
detection work to QEMU, or the kernel has to be part of the detection 
(e.g. the kick before DRIVER_OK thing we have to duplicate the tracking 
effort in QEMU) as well. Let me take a further look and get back.

Meanwhile, I'll check internally to see if a legacy only model would 
work. Thanks.

Thanks,
-Siwei


>
>
>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
>>>>>>>>> Well, although I think adding an ioctl is doable, may I
>>>>>>>>> know what the use
>>>>>>>>> case there will be for kernel to leverage such info
>>>>>>>>> directly? Is there a
>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>>>>>>> differentiation (legacy v.s. modern) needed?
>>>>>>>> BTW a good API could be
>>>>>>>>
>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>
>>>>>>>> we did it per vring but maybe that was a mistake ...
>>>>>>> Actually, I wonder whether it's good time to just not support
>>>>>>> legacy driver
>>>>>>> for vDPA. Consider:
>>>>>>>
>>>>>>> 1) It's definition is no-normative
>>>>>>> 2) A lot of budren of codes
>>>>>>>
>>>>>>> So qemu can still present the legacy device since the config
>>>>>>> space or other
>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
>>>>>>> accessed by
>>>>>>> guest directly. Qemu can do the endian conversion when necessary
>>>>>>> in this
>>>>>>> case?
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>> Overall I would be fine with this approach but we need to avoid breaking
>>>>>> working userspace, qemu releases with vdpa support are out there and
>>>>>> seem to work for people. Any changes need to take that into account
>>>>>> and document compatibility concerns.
>>>>> Agree, let me check.
>>>>>
>>>>>
>>>>>>     I note that any hardware
>>>>>> implementation is already broken for legacy except on platforms with
>>>>>> strong ordering which might be helpful in reducing the scope.
>>>>> Yes.
>>>>>
>>>>> Thanks
>>>>>
>>>>>

Jason Wang Dec. 15, 2021, 2:06 a.m. UTC | #75

On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> > On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> >>
> >> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> >>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> >>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> >>>> it ended up with. I have the following questions,
> >>>>
> >>>> 1. legacy guest support: from the past conversations it doesn't seem the
> >>>> support will be completely dropped from the table, is my understanding
> >>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> >>>> which is backed by the spec at
> >>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> >>>> if there's request/need to support wilder legacy virtio versions earlier
> >>>> beyond.
> >>> I personally feel it's less work to add in kernel than try to
> >>> work around it in userspace. Jason feels differently.
> >>> Maybe post the patches and this will prove to Jason it's not
> >>> too terrible?
> >> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> >> layout level and is limited to x86 only, there should be easy way out.
> > Note a subtle difference: what matters is that guest, not host is x86.
> > Matters for emulators which might reorder memory accesses.
> > I guess this enforcement belongs in QEMU then?
> Right, I mean to get started, the initial guest driver support and the
> corresponding QEMU support for transitional vdpa backend can be limited
> to x86 guest/host only. Since the config space is emulated in QEMU, I
> suppose it's not hard to enforce in QEMU.

It's more than just config space, most devices have headers before the buffer.

> QEMU can drive GET_LEGACY,
> GET_ENDIAN et al ioctls in advance to get the capability from the
> individual vendor driver. For that, we need another negotiation protocol
> similar to vhost_user's protocol_features between the vdpa kernel and
> QEMU, way before the guest driver is ever probed and its feature
> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> from the device, but we can assume weak ordering for legacy at this
> point (x86 only)?

I'm lost here, we have get_features() so:

1) VERSION_1 means the device uses LE if provided, otherwise natvie
2) ORDER_PLATFORM means device requires platform ordering

Any reason for having a new API for this?

>
> >
> >> I
> >> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> >> 0.95 support, it seems all the ingredient had been there already dated back
> >> to the DPDK days. The only major thing limiting is in the vDPA software that
> >> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> >> a few DMA setup ops, which is virtio 1.0 only.
> >>
> >>>> 2. suppose some form of legacy guest support needs to be there, how do we
> >>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
> >>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
> >>>> of vdpa_get_config() to vdpa_set_config().
> >>>>
> >>>>           /*
> >>>>            * Config accesses aren't supposed to trigger before features are
> >>>> set.
> >>>>            * If it does happen we assume a legacy guest.
> >>>>            */
> >>>>           if (!vdev->features_valid)
> >>>>                   vdpa_set_features(vdev, 0);
> >>>>           ops->get_config(vdev, offset, buf, len);
> >>>>
> >>>> I can post a patch to fix 2) if there's consensus already reached.
> >>>>
> >>>> Thanks,
> >>>> -Siwei
> >>> I'm not sure how important it is to change that.
> >>> In any case it only affects transitional devices, right?
> >>> Legacy only should not care ...
> >> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> >> modern one in a transitional device model rather than being legacy only.
> >> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> >> guests without having to reconfigure. Or are you suggesting limit to legacy
> >> only at the time of vdpa creation would simplify the implementation a lot?
> >>
> >> Thanks,
> >> -Siwei
> >
> > I don't know for sure. Take a look at the work Halil was doing
> > to try and support transitional devices with BE guests.
> Hmmm, we can have those endianness ioctls defined but the initial QEMU
> implementation can be started to support x86 guest/host with little
> endian and weak memory ordering first. The real trick is to detect
> legacy guest - I am not sure if it's feasible to shift all the legacy
> detection work to QEMU, or the kernel has to be part of the detection
> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> effort in QEMU) as well. Let me take a further look and get back.

Michael may think differently but I think doing this in Qemu is much easier.

Thanks



>
> Meanwhile, I'll check internally to see if a legacy only model would
> work. Thanks.
>
> Thanks,
> -Siwei
>
>
> >
> >
> >>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
> >>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> >>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> >>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> >>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> >>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
> >>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
> >>>>>>>>> Well, although I think adding an ioctl is doable, may I
> >>>>>>>>> know what the use
> >>>>>>>>> case there will be for kernel to leverage such info
> >>>>>>>>> directly? Is there a
> >>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
> >>>>>>>>> differentiation (legacy v.s. modern) needed?
> >>>>>>>> BTW a good API could be
> >>>>>>>>
> >>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>
> >>>>>>>> we did it per vring but maybe that was a mistake ...
> >>>>>>> Actually, I wonder whether it's good time to just not support
> >>>>>>> legacy driver
> >>>>>>> for vDPA. Consider:
> >>>>>>>
> >>>>>>> 1) It's definition is no-normative
> >>>>>>> 2) A lot of budren of codes
> >>>>>>>
> >>>>>>> So qemu can still present the legacy device since the config
> >>>>>>> space or other
> >>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
> >>>>>>> accessed by
> >>>>>>> guest directly. Qemu can do the endian conversion when necessary
> >>>>>>> in this
> >>>>>>> case?
> >>>>>>>
> >>>>>>> Thanks
> >>>>>>>
> >>>>>> Overall I would be fine with this approach but we need to avoid breaking
> >>>>>> working userspace, qemu releases with vdpa support are out there and
> >>>>>> seem to work for people. Any changes need to take that into account
> >>>>>> and document compatibility concerns.
> >>>>> Agree, let me check.
> >>>>>
> >>>>>
> >>>>>>     I note that any hardware
> >>>>>> implementation is already broken for legacy except on platforms with
> >>>>>> strong ordering which might be helpful in reducing the scope.
> >>>>> Yes.
> >>>>>
> >>>>> Thanks
> >>>>>
> >>>>>
>

Si-Wei Liu Dec. 15, 2021, 8:52 p.m. UTC | #76

On 12/14/2021 6:06 PM, Jason Wang wrote:
> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
>>
>>
>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
>>>>>> it ended up with. I have the following questions,
>>>>>>
>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
>>>>>> support will be completely dropped from the table, is my understanding
>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
>>>>>> which is backed by the spec at
>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
>>>>>> if there's request/need to support wilder legacy virtio versions earlier
>>>>>> beyond.
>>>>> I personally feel it's less work to add in kernel than try to
>>>>> work around it in userspace. Jason feels differently.
>>>>> Maybe post the patches and this will prove to Jason it's not
>>>>> too terrible?
>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
>>>> layout level and is limited to x86 only, there should be easy way out.
>>> Note a subtle difference: what matters is that guest, not host is x86.
>>> Matters for emulators which might reorder memory accesses.
>>> I guess this enforcement belongs in QEMU then?
>> Right, I mean to get started, the initial guest driver support and the
>> corresponding QEMU support for transitional vdpa backend can be limited
>> to x86 guest/host only. Since the config space is emulated in QEMU, I
>> suppose it's not hard to enforce in QEMU.
> It's more than just config space, most devices have headers before the buffer.
The ordering in datapath (data VQs) would have to rely on vendor's 
support. Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w 
vendor nowadays can/should well support the case when ORDER_PLATFORM is 
not acked by the driver (actually this feature is filtered out by the 
QEMU vhost-vdpa driver today), even with v1.0 spec conforming and modern 
only vDPA device. The control VQ is implemented in software in the 
kernel, which can be easily accommodated/fixed when needed.

>
>> QEMU can drive GET_LEGACY,
>> GET_ENDIAN et al ioctls in advance to get the capability from the
>> individual vendor driver. For that, we need another negotiation protocol
>> similar to vhost_user's protocol_features between the vdpa kernel and
>> QEMU, way before the guest driver is ever probed and its feature
>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
>> from the device, but we can assume weak ordering for legacy at this
>> point (x86 only)?
> I'm lost here, we have get_features() so:
I assume here you refer to get_device_features() that Eli just changed 
the name.
>
> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> 2) ORDER_PLATFORM means device requires platform ordering
>
> Any reason for having a new API for this?
Are you going to enforce all vDPA hardware vendors to support the 
transitional model for legacy guest? meaning guest not acknowledging 
VERSION_1 would use the legacy interfaces captured in the spec section 
7.4 (regarding ring layout, native endianness, message framing, vq 
alignment of 4096, 32bit feature, no features_ok bit in status, IO port 
interface i.e. all the things) instead? Noted we don't yet have a 
set_device_features() that allows the vdpa device to tell whether it is 
operating in transitional or modern-only mode. For software virtio, all 
support for the legacy part in a transitional model has been built up 
there already, however, it's not easy for vDPA vendors to implement all 
the requirements for an all-or-nothing legacy guest support (big endian 
guest for example). To these vendors, the legacy support within a 
transitional model is more of feature to them and it's best to leave 
some flexibility for them to implement partial support for legacy. That 
in turn calls out the need for a vhost-user protocol feature like 
negotiation API that can prohibit those unsupported guest setups to as 
early as backend_init before launching the VM.


>
>>>> I
>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
>>>> 0.95 support, it seems all the ingredient had been there already dated back
>>>> to the DPDK days. The only major thing limiting is in the vDPA software that
>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
>>>> a few DMA setup ops, which is virtio 1.0 only.
>>>>
>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
>>>>>> of vdpa_get_config() to vdpa_set_config().
>>>>>>
>>>>>>            /*
>>>>>>             * Config accesses aren't supposed to trigger before features are
>>>>>> set.
>>>>>>             * If it does happen we assume a legacy guest.
>>>>>>             */
>>>>>>            if (!vdev->features_valid)
>>>>>>                    vdpa_set_features(vdev, 0);
>>>>>>            ops->get_config(vdev, offset, buf, len);
>>>>>>
>>>>>> I can post a patch to fix 2) if there's consensus already reached.
>>>>>>
>>>>>> Thanks,
>>>>>> -Siwei
>>>>> I'm not sure how important it is to change that.
>>>>> In any case it only affects transitional devices, right?
>>>>> Legacy only should not care ...
>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
>>>> modern one in a transitional device model rather than being legacy only.
>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
>>>> guests without having to reconfigure. Or are you suggesting limit to legacy
>>>> only at the time of vdpa creation would simplify the implementation a lot?
>>>>
>>>> Thanks,
>>>> -Siwei
>>> I don't know for sure. Take a look at the work Halil was doing
>>> to try and support transitional devices with BE guests.
>> Hmmm, we can have those endianness ioctls defined but the initial QEMU
>> implementation can be started to support x86 guest/host with little
>> endian and weak memory ordering first. The real trick is to detect
>> legacy guest - I am not sure if it's feasible to shift all the legacy
>> detection work to QEMU, or the kernel has to be part of the detection
>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
>> effort in QEMU) as well. Let me take a further look and get back.
> Michael may think differently but I think doing this in Qemu is much easier.
I think the key is whether we position emulating legacy interfaces in 
QEMU doing translation on top of a v1.0 modern-only device in the 
kernel, or we allow vdpa core (or you can say vhost-vdpa) and vendor 
driver to support a transitional model in the kernel that is able to 
work for both v0.95 and v1.0 drivers, with some slight aid from QEMU for 
detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for 
the former we still rely on vendor for a performant data vqs 
implementation, leaving the question to what it may end up eventually in 
the kernel is effectively the latter).

Thanks,
-Siwei

>
> Thanks
>
>
>
>> Meanwhile, I'll check internally to see if a legacy only model would
>> work. Thanks.
>>
>> Thanks,
>> -Siwei
>>
>>
>>>
>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
>>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
>>>>>>>>>>> know what the use
>>>>>>>>>>> case there will be for kernel to leverage such info
>>>>>>>>>>> directly? Is there a
>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>>>>>>>>> differentiation (legacy v.s. modern) needed?
>>>>>>>>>> BTW a good API could be
>>>>>>>>>>
>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>>>
>>>>>>>>>> we did it per vring but maybe that was a mistake ...
>>>>>>>>> Actually, I wonder whether it's good time to just not support
>>>>>>>>> legacy driver
>>>>>>>>> for vDPA. Consider:
>>>>>>>>>
>>>>>>>>> 1) It's definition is no-normative
>>>>>>>>> 2) A lot of budren of codes
>>>>>>>>>
>>>>>>>>> So qemu can still present the legacy device since the config
>>>>>>>>> space or other
>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
>>>>>>>>> accessed by
>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
>>>>>>>>> in this
>>>>>>>>> case?
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>>
>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
>>>>>>>> working userspace, qemu releases with vdpa support are out there and
>>>>>>>> seem to work for people. Any changes need to take that into account
>>>>>>>> and document compatibility concerns.
>>>>>>> Agree, let me check.
>>>>>>>
>>>>>>>
>>>>>>>>      I note that any hardware
>>>>>>>> implementation is already broken for legacy except on platforms with
>>>>>>>> strong ordering which might be helpful in reducing the scope.
>>>>>>> Yes.
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>>>

Michael S. Tsirkin Dec. 15, 2021, 9:33 p.m. UTC | #77

On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
> 
> 
> On 12/14/2021 6:06 PM, Jason Wang wrote:
> > On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
> > > 
> > > 
> > > On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> > > > On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> > > > > On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> > > > > > On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > > > > > > Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > > > > > > it ended up with. I have the following questions,
> > > > > > > 
> > > > > > > 1. legacy guest support: from the past conversations it doesn't seem the
> > > > > > > support will be completely dropped from the table, is my understanding
> > > > > > > correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > > > > > > which is backed by the spec at
> > > > > > > https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> > > > > > > if there's request/need to support wilder legacy virtio versions earlier
> > > > > > > beyond.
> > > > > > I personally feel it's less work to add in kernel than try to
> > > > > > work around it in userspace. Jason feels differently.
> > > > > > Maybe post the patches and this will prove to Jason it's not
> > > > > > too terrible?
> > > > > I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> > > > > layout level and is limited to x86 only, there should be easy way out.
> > > > Note a subtle difference: what matters is that guest, not host is x86.
> > > > Matters for emulators which might reorder memory accesses.
> > > > I guess this enforcement belongs in QEMU then?
> > > Right, I mean to get started, the initial guest driver support and the
> > > corresponding QEMU support for transitional vdpa backend can be limited
> > > to x86 guest/host only. Since the config space is emulated in QEMU, I
> > > suppose it's not hard to enforce in QEMU.
> > It's more than just config space, most devices have headers before the buffer.
> The ordering in datapath (data VQs) would have to rely on vendor's support.
> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
> can/should well support the case when ORDER_PLATFORM is not acked by the
> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
> today), even with v1.0 spec conforming and modern only vDPA device. The
> control VQ is implemented in software in the kernel, which can be easily
> accommodated/fixed when needed.
> 
> > 
> > > QEMU can drive GET_LEGACY,
> > > GET_ENDIAN et al ioctls in advance to get the capability from the
> > > individual vendor driver. For that, we need another negotiation protocol
> > > similar to vhost_user's protocol_features between the vdpa kernel and
> > > QEMU, way before the guest driver is ever probed and its feature
> > > negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> > > from the device, but we can assume weak ordering for legacy at this
> > > point (x86 only)?
> > I'm lost here, we have get_features() so:
> I assume here you refer to get_device_features() that Eli just changed the
> name.
> > 
> > 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> > 2) ORDER_PLATFORM means device requires platform ordering
> > 
> > Any reason for having a new API for this?
> Are you going to enforce all vDPA hardware vendors to support the
> transitional model for legacy guest? meaning guest not acknowledging
> VERSION_1 would use the legacy interfaces captured in the spec section 7.4
> (regarding ring layout, native endianness, message framing, vq alignment of
> 4096, 32bit feature, no features_ok bit in status, IO port interface i.e.
> all the things) instead? Noted we don't yet have a set_device_features()
> that allows the vdpa device to tell whether it is operating in transitional
> or modern-only mode. For software virtio, all support for the legacy part in
> a transitional model has been built up there already, however, it's not easy
> for vDPA vendors to implement all the requirements for an all-or-nothing
> legacy guest support (big endian guest for example). To these vendors, the
> legacy support within a transitional model is more of feature to them and
> it's best to leave some flexibility for them to implement partial support
> for legacy. That in turn calls out the need for a vhost-user protocol
> feature like negotiation API that can prohibit those unsupported guest
> setups to as early as backend_init before launching the VM.

Right. Of note is the fact that it's a spec bug which I
hope yet to fix, though due to existing guest code the
fix won't be complete.

WRT ioctls, One thing we can do though is abuse set_features
where it's called by QEMU early on with just the VERSION_1
bit set, to distinguish between legacy and modern
interface. This before config space accesses and FEATURES_OK.

Halil has been working on this, pls take a look and maybe help him out.

> 
> > 
> > > > > I
> > > > > checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> > > > > 0.95 support, it seems all the ingredient had been there already dated back
> > > > > to the DPDK days. The only major thing limiting is in the vDPA software that
> > > > > the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> > > > > a few DMA setup ops, which is virtio 1.0 only.
> > > > > 
> > > > > > > 2. suppose some form of legacy guest support needs to be there, how do we
> > > > > > > deal with the bogus assumption below in vdpa_get_config() in the short term?
> > > > > > > It looks one of the intuitive fix is to move the vdpa_set_features call out
> > > > > > > of vdpa_get_config() to vdpa_set_config().
> > > > > > > 
> > > > > > >            /*
> > > > > > >             * Config accesses aren't supposed to trigger before features are
> > > > > > > set.
> > > > > > >             * If it does happen we assume a legacy guest.
> > > > > > >             */
> > > > > > >            if (!vdev->features_valid)
> > > > > > >                    vdpa_set_features(vdev, 0);
> > > > > > >            ops->get_config(vdev, offset, buf, len);
> > > > > > > 
> > > > > > > I can post a patch to fix 2) if there's consensus already reached.
> > > > > > > 
> > > > > > > Thanks,
> > > > > > > -Siwei
> > > > > > I'm not sure how important it is to change that.
> > > > > > In any case it only affects transitional devices, right?
> > > > > > Legacy only should not care ...
> > > > > Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> > > > > modern one in a transitional device model rather than being legacy only.
> > > > > That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> > > > > guests without having to reconfigure. Or are you suggesting limit to legacy
> > > > > only at the time of vdpa creation would simplify the implementation a lot?
> > > > > 
> > > > > Thanks,
> > > > > -Siwei
> > > > I don't know for sure. Take a look at the work Halil was doing
> > > > to try and support transitional devices with BE guests.
> > > Hmmm, we can have those endianness ioctls defined but the initial QEMU
> > > implementation can be started to support x86 guest/host with little
> > > endian and weak memory ordering first. The real trick is to detect
> > > legacy guest - I am not sure if it's feasible to shift all the legacy
> > > detection work to QEMU, or the kernel has to be part of the detection
> > > (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> > > effort in QEMU) as well. Let me take a further look and get back.
> > Michael may think differently but I think doing this in Qemu is much easier.
> I think the key is whether we position emulating legacy interfaces in QEMU
> doing translation on top of a v1.0 modern-only device in the kernel, or we
> allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a
> transitional model in the kernel that is able to work for both v0.95 and
> v1.0 drivers, with some slight aid from QEMU for
> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the
> former we still rely on vendor for a performant data vqs implementation,
> leaving the question to what it may end up eventually in the kernel is
> effectively the latter).
> 
> Thanks,
> -Siwei


My suggestion is post the kernel patches, and we can evaluate
how much work they are.

> > 
> > Thanks
> > 
> > 
> > 
> > > Meanwhile, I'll check internally to see if a legacy only model would
> > > work. Thanks.
> > > 
> > > Thanks,
> > > -Siwei
> > > 
> > > 
> > > > 
> > > > > > > On 3/2/2021 2:53 AM, Jason Wang wrote:
> > > > > > > > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > > > > > > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > > > > > > > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > > > > > > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > > > > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > > > > > > > > > > the kernel that it's a legacy guest. Ugh :(
> > > > > > > > > > > > Well, although I think adding an ioctl is doable, may I
> > > > > > > > > > > > know what the use
> > > > > > > > > > > > case there will be for kernel to leverage such info
> > > > > > > > > > > > directly? Is there a
> > > > > > > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > > > > > > > > > > differentiation (legacy v.s. modern) needed?
> > > > > > > > > > > BTW a good API could be
> > > > > > > > > > > 
> > > > > > > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > > > > > 
> > > > > > > > > > > we did it per vring but maybe that was a mistake ...
> > > > > > > > > > Actually, I wonder whether it's good time to just not support
> > > > > > > > > > legacy driver
> > > > > > > > > > for vDPA. Consider:
> > > > > > > > > > 
> > > > > > > > > > 1) It's definition is no-normative
> > > > > > > > > > 2) A lot of budren of codes
> > > > > > > > > > 
> > > > > > > > > > So qemu can still present the legacy device since the config
> > > > > > > > > > space or other
> > > > > > > > > > stuffs that is presented by vhost-vDPA is not expected to be
> > > > > > > > > > accessed by
> > > > > > > > > > guest directly. Qemu can do the endian conversion when necessary
> > > > > > > > > > in this
> > > > > > > > > > case?
> > > > > > > > > > 
> > > > > > > > > > Thanks
> > > > > > > > > > 
> > > > > > > > > Overall I would be fine with this approach but we need to avoid breaking
> > > > > > > > > working userspace, qemu releases with vdpa support are out there and
> > > > > > > > > seem to work for people. Any changes need to take that into account
> > > > > > > > > and document compatibility concerns.
> > > > > > > > Agree, let me check.
> > > > > > > > 
> > > > > > > > 
> > > > > > > > >      I note that any hardware
> > > > > > > > > implementation is already broken for legacy except on platforms with
> > > > > > > > > strong ordering which might be helpful in reducing the scope.
> > > > > > > > Yes.
> > > > > > > > 
> > > > > > > > Thanks
> > > > > > > > 
> > > > > > > >

Si-Wei Liu Dec. 16, 2021, 2:01 a.m. UTC | #78

On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:
> On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
>>
>> On 12/14/2021 6:06 PM, Jason Wang wrote:
>>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
>>>>
>>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
>>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
>>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
>>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
>>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
>>>>>>>> it ended up with. I have the following questions,
>>>>>>>>
>>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
>>>>>>>> support will be completely dropped from the table, is my understanding
>>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
>>>>>>>> which is backed by the spec at
>>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
>>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
>>>>>>>> beyond.
>>>>>>> I personally feel it's less work to add in kernel than try to
>>>>>>> work around it in userspace. Jason feels differently.
>>>>>>> Maybe post the patches and this will prove to Jason it's not
>>>>>>> too terrible?
>>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
>>>>>> layout level and is limited to x86 only, there should be easy way out.
>>>>> Note a subtle difference: what matters is that guest, not host is x86.
>>>>> Matters for emulators which might reorder memory accesses.
>>>>> I guess this enforcement belongs in QEMU then?
>>>> Right, I mean to get started, the initial guest driver support and the
>>>> corresponding QEMU support for transitional vdpa backend can be limited
>>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
>>>> suppose it's not hard to enforce in QEMU.
>>> It's more than just config space, most devices have headers before the buffer.
>> The ordering in datapath (data VQs) would have to rely on vendor's support.
>> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
>> can/should well support the case when ORDER_PLATFORM is not acked by the
>> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
>> today), even with v1.0 spec conforming and modern only vDPA device. The
>> control VQ is implemented in software in the kernel, which can be easily
>> accommodated/fixed when needed.
>>
>>>> QEMU can drive GET_LEGACY,
>>>> GET_ENDIAN et al ioctls in advance to get the capability from the
>>>> individual vendor driver. For that, we need another negotiation protocol
>>>> similar to vhost_user's protocol_features between the vdpa kernel and
>>>> QEMU, way before the guest driver is ever probed and its feature
>>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
>>>> from the device, but we can assume weak ordering for legacy at this
>>>> point (x86 only)?
>>> I'm lost here, we have get_features() so:
>> I assume here you refer to get_device_features() that Eli just changed the
>> name.
>>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
>>> 2) ORDER_PLATFORM means device requires platform ordering
>>>
>>> Any reason for having a new API for this?
>> Are you going to enforce all vDPA hardware vendors to support the
>> transitional model for legacy guest? meaning guest not acknowledging
>> VERSION_1 would use the legacy interfaces captured in the spec section 7.4
>> (regarding ring layout, native endianness, message framing, vq alignment of
>> 4096, 32bit feature, no features_ok bit in status, IO port interface i.e.
>> all the things) instead? Noted we don't yet have a set_device_features()
>> that allows the vdpa device to tell whether it is operating in transitional
>> or modern-only mode. For software virtio, all support for the legacy part in
>> a transitional model has been built up there already, however, it's not easy
>> for vDPA vendors to implement all the requirements for an all-or-nothing
>> legacy guest support (big endian guest for example). To these vendors, the
>> legacy support within a transitional model is more of feature to them and
>> it's best to leave some flexibility for them to implement partial support
>> for legacy. That in turn calls out the need for a vhost-user protocol
>> feature like negotiation API that can prohibit those unsupported guest
>> setups to as early as backend_init before launching the VM.
> Right. Of note is the fact that it's a spec bug which I
> hope yet to fix, though due to existing guest code the
> fix won't be complete.
I thought at one point you pointed out to me that the spec does allow 
config space read before claiming features_ok, and only config write 
before features_ok is prohibited. I haven't read up the full thread of 
Halil's VERSION_1 for transitional big endian device yet, but what is 
the spec bug you hope to fix?

>
> WRT ioctls, One thing we can do though is abuse set_features
> where it's called by QEMU early on with just the VERSION_1
> bit set, to distinguish between legacy and modern
> interface. This before config space accesses and FEATURES_OK.
>
> Halil has been working on this, pls take a look and maybe help him out.
Interesting thread, am reading now and see how I may leverage or help there.

>>>>>> I
>>>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
>>>>>> 0.95 support, it seems all the ingredient had been there already dated back
>>>>>> to the DPDK days. The only major thing limiting is in the vDPA software that
>>>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
>>>>>> a few DMA setup ops, which is virtio 1.0 only.
>>>>>>
>>>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
>>>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
>>>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
>>>>>>>> of vdpa_get_config() to vdpa_set_config().
>>>>>>>>
>>>>>>>>             /*
>>>>>>>>              * Config accesses aren't supposed to trigger before features are
>>>>>>>> set.
>>>>>>>>              * If it does happen we assume a legacy guest.
>>>>>>>>              */
>>>>>>>>             if (!vdev->features_valid)
>>>>>>>>                     vdpa_set_features(vdev, 0);
>>>>>>>>             ops->get_config(vdev, offset, buf, len);
>>>>>>>>
>>>>>>>> I can post a patch to fix 2) if there's consensus already reached.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> -Siwei
>>>>>>> I'm not sure how important it is to change that.
>>>>>>> In any case it only affects transitional devices, right?
>>>>>>> Legacy only should not care ...
>>>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
>>>>>> modern one in a transitional device model rather than being legacy only.
>>>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
>>>>>> guests without having to reconfigure. Or are you suggesting limit to legacy
>>>>>> only at the time of vdpa creation would simplify the implementation a lot?
>>>>>>
>>>>>> Thanks,
>>>>>> -Siwei
>>>>> I don't know for sure. Take a look at the work Halil was doing
>>>>> to try and support transitional devices with BE guests.
>>>> Hmmm, we can have those endianness ioctls defined but the initial QEMU
>>>> implementation can be started to support x86 guest/host with little
>>>> endian and weak memory ordering first. The real trick is to detect
>>>> legacy guest - I am not sure if it's feasible to shift all the legacy
>>>> detection work to QEMU, or the kernel has to be part of the detection
>>>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
>>>> effort in QEMU) as well. Let me take a further look and get back.
>>> Michael may think differently but I think doing this in Qemu is much easier.
>> I think the key is whether we position emulating legacy interfaces in QEMU
>> doing translation on top of a v1.0 modern-only device in the kernel, or we
>> allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a
>> transitional model in the kernel that is able to work for both v0.95 and
>> v1.0 drivers, with some slight aid from QEMU for
>> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the
>> former we still rely on vendor for a performant data vqs implementation,
>> leaving the question to what it may end up eventually in the kernel is
>> effectively the latter).
>>
>> Thanks,
>> -Siwei
>
> My suggestion is post the kernel patches, and we can evaluate
> how much work they are.
Thanks for the feedback. I will take some read then get back, probably 
after the winter break. Stay tuned.

Thanks,
-Siwei

>
>>> Thanks
>>>
>>>
>>>
>>>> Meanwhile, I'll check internally to see if a legacy only model would
>>>> work. Thanks.
>>>>
>>>> Thanks,
>>>> -Siwei
>>>>
>>>>
>>>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
>>>>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
>>>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>>>>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
>>>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
>>>>>>>>>>>>> know what the use
>>>>>>>>>>>>> case there will be for kernel to leverage such info
>>>>>>>>>>>>> directly? Is there a
>>>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>>>>>>>>>>> differentiation (legacy v.s. modern) needed?
>>>>>>>>>>>> BTW a good API could be
>>>>>>>>>>>>
>>>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>>>>>
>>>>>>>>>>>> we did it per vring but maybe that was a mistake ...
>>>>>>>>>>> Actually, I wonder whether it's good time to just not support
>>>>>>>>>>> legacy driver
>>>>>>>>>>> for vDPA. Consider:
>>>>>>>>>>>
>>>>>>>>>>> 1) It's definition is no-normative
>>>>>>>>>>> 2) A lot of budren of codes
>>>>>>>>>>>
>>>>>>>>>>> So qemu can still present the legacy device since the config
>>>>>>>>>>> space or other
>>>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
>>>>>>>>>>> accessed by
>>>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
>>>>>>>>>>> in this
>>>>>>>>>>> case?
>>>>>>>>>>>
>>>>>>>>>>> Thanks
>>>>>>>>>>>
>>>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
>>>>>>>>>> working userspace, qemu releases with vdpa support are out there and
>>>>>>>>>> seem to work for people. Any changes need to take that into account
>>>>>>>>>> and document compatibility concerns.
>>>>>>>>> Agree, let me check.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>       I note that any hardware
>>>>>>>>>> implementation is already broken for legacy except on platforms with
>>>>>>>>>> strong ordering which might be helpful in reducing the scope.
>>>>>>>>> Yes.
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>>
>>>>>>>>>

Jason Wang Dec. 16, 2021, 2:53 a.m. UTC | #79

On Thu, Dec 16, 2021 at 10:02 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:
> > On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
> >>
> >> On 12/14/2021 6:06 PM, Jason Wang wrote:
> >>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>
> >>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> >>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> >>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> >>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> >>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> >>>>>>>> it ended up with. I have the following questions,
> >>>>>>>>
> >>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
> >>>>>>>> support will be completely dropped from the table, is my understanding
> >>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> >>>>>>>> which is backed by the spec at
> >>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> >>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
> >>>>>>>> beyond.
> >>>>>>> I personally feel it's less work to add in kernel than try to
> >>>>>>> work around it in userspace. Jason feels differently.
> >>>>>>> Maybe post the patches and this will prove to Jason it's not
> >>>>>>> too terrible?
> >>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> >>>>>> layout level and is limited to x86 only, there should be easy way out.
> >>>>> Note a subtle difference: what matters is that guest, not host is x86.
> >>>>> Matters for emulators which might reorder memory accesses.
> >>>>> I guess this enforcement belongs in QEMU then?
> >>>> Right, I mean to get started, the initial guest driver support and the
> >>>> corresponding QEMU support for transitional vdpa backend can be limited
> >>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
> >>>> suppose it's not hard to enforce in QEMU.
> >>> It's more than just config space, most devices have headers before the buffer.
> >> The ordering in datapath (data VQs) would have to rely on vendor's support.
> >> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
> >> can/should well support the case when ORDER_PLATFORM is not acked by the
> >> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
> >> today), even with v1.0 spec conforming and modern only vDPA device. The
> >> control VQ is implemented in software in the kernel, which can be easily
> >> accommodated/fixed when needed.
> >>
> >>>> QEMU can drive GET_LEGACY,
> >>>> GET_ENDIAN et al ioctls in advance to get the capability from the
> >>>> individual vendor driver. For that, we need another negotiation protocol
> >>>> similar to vhost_user's protocol_features between the vdpa kernel and
> >>>> QEMU, way before the guest driver is ever probed and its feature
> >>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> >>>> from the device, but we can assume weak ordering for legacy at this
> >>>> point (x86 only)?
> >>> I'm lost here, we have get_features() so:
> >> I assume here you refer to get_device_features() that Eli just changed the
> >> name.
> >>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> >>> 2) ORDER_PLATFORM means device requires platform ordering
> >>>
> >>> Any reason for having a new API for this?
> >> Are you going to enforce all vDPA hardware vendors to support the
> >> transitional model for legacy guest?

Do we really have other choices?

I suspect the legacy device is never implemented by any vendor:

1) no virtio way to detect host endian
2) bypass IOMMU with translated requests
3) PIO port

Yes we have enp_vdpa, but it's more like a "transitional device" for
legacy only guests.

> meaning guest not acknowledging
> >> VERSION_1 would use the legacy interfaces captured in the spec section 7.4
> >> (regarding ring layout, native endianness, message framing, vq alignment of
> >> 4096, 32bit feature, no features_ok bit in status, IO port interface i.e.
> >> all the things) instead?

Note that we only care about the datapath, control path is mediated anyhow.

So feature_ok and IO port isn't an issue. The rest looks like a must
for the hardware.

> Noted we don't yet have a set_device_features()
> >> that allows the vdpa device to tell whether it is operating in transitional
> >> or modern-only mode.

So the device feature should be provisioned via the netlink protocol.
And what we want is not "set_device_feature()" but
"set_device_mandatory_feautre()", then the parent can choose to fail
the negotiation when VERSION_1 is not negotiated. Qemu then knows for
sure it talks to a transitional device or modern only device.

Thanks

> For software virtio, all support for the legacy part in
> >> a transitional model has been built up there already, however, it's not easy
> >> for vDPA vendors to implement all the requirements for an all-or-nothing
> >> legacy guest support (big endian guest for example). To these vendors, the
> >> legacy support within a transitional model is more of feature to them and
> >> it's best to leave some flexibility for them to implement partial support
> >> for legacy. That in turn calls out the need for a vhost-user protocol
> >> feature like negotiation API that can prohibit those unsupported guest
> >> setups to as early as backend_init before launching the VM.
> > Right. Of note is the fact that it's a spec bug which I
> > hope yet to fix, though due to existing guest code the
> > fix won't be complete.
> I thought at one point you pointed out to me that the spec does allow
> config space read before claiming features_ok, and only config write
> before features_ok is prohibited. I haven't read up the full thread of
> Halil's VERSION_1 for transitional big endian device yet, but what is
> the spec bug you hope to fix?
>
> >
> > WRT ioctls, One thing we can do though is abuse set_features
> > where it's called by QEMU early on with just the VERSION_1
> > bit set, to distinguish between legacy and modern
> > interface. This before config space accesses and FEATURES_OK.
> >
> > Halil has been working on this, pls take a look and maybe help him out.
> Interesting thread, am reading now and see how I may leverage or help there.
>
> >>>>>> I
> >>>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> >>>>>> 0.95 support, it seems all the ingredient had been there already dated back
> >>>>>> to the DPDK days. The only major thing limiting is in the vDPA software that
> >>>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> >>>>>> a few DMA setup ops, which is virtio 1.0 only.
> >>>>>>
> >>>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
> >>>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
> >>>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
> >>>>>>>> of vdpa_get_config() to vdpa_set_config().
> >>>>>>>>
> >>>>>>>>             /*
> >>>>>>>>              * Config accesses aren't supposed to trigger before features are
> >>>>>>>> set.
> >>>>>>>>              * If it does happen we assume a legacy guest.
> >>>>>>>>              */
> >>>>>>>>             if (!vdev->features_valid)
> >>>>>>>>                     vdpa_set_features(vdev, 0);
> >>>>>>>>             ops->get_config(vdev, offset, buf, len);
> >>>>>>>>
> >>>>>>>> I can post a patch to fix 2) if there's consensus already reached.
> >>>>>>>>
> >>>>>>>> Thanks,
> >>>>>>>> -Siwei
> >>>>>>> I'm not sure how important it is to change that.
> >>>>>>> In any case it only affects transitional devices, right?
> >>>>>>> Legacy only should not care ...
> >>>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> >>>>>> modern one in a transitional device model rather than being legacy only.
> >>>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> >>>>>> guests without having to reconfigure. Or are you suggesting limit to legacy
> >>>>>> only at the time of vdpa creation would simplify the implementation a lot?
> >>>>>>
> >>>>>> Thanks,
> >>>>>> -Siwei
> >>>>> I don't know for sure. Take a look at the work Halil was doing
> >>>>> to try and support transitional devices with BE guests.
> >>>> Hmmm, we can have those endianness ioctls defined but the initial QEMU
> >>>> implementation can be started to support x86 guest/host with little
> >>>> endian and weak memory ordering first. The real trick is to detect
> >>>> legacy guest - I am not sure if it's feasible to shift all the legacy
> >>>> detection work to QEMU, or the kernel has to be part of the detection
> >>>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> >>>> effort in QEMU) as well. Let me take a further look and get back.
> >>> Michael may think differently but I think doing this in Qemu is much easier.
> >> I think the key is whether we position emulating legacy interfaces in QEMU
> >> doing translation on top of a v1.0 modern-only device in the kernel, or we
> >> allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a
> >> transitional model in the kernel that is able to work for both v0.95 and
> >> v1.0 drivers, with some slight aid from QEMU for
> >> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the
> >> former we still rely on vendor for a performant data vqs implementation,
> >> leaving the question to what it may end up eventually in the kernel is
> >> effectively the latter).
> >>
> >> Thanks,
> >> -Siwei
> >
> > My suggestion is post the kernel patches, and we can evaluate
> > how much work they are.
> Thanks for the feedback. I will take some read then get back, probably
> after the winter break. Stay tuned.
>
> Thanks,
> -Siwei
>
> >
> >>> Thanks
> >>>
> >>>
> >>>
> >>>> Meanwhile, I'll check internally to see if a legacy only model would
> >>>> work. Thanks.
> >>>>
> >>>> Thanks,
> >>>> -Siwei
> >>>>
> >>>>
> >>>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
> >>>>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> >>>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> >>>>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> >>>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> >>>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
> >>>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
> >>>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
> >>>>>>>>>>>>> know what the use
> >>>>>>>>>>>>> case there will be for kernel to leverage such info
> >>>>>>>>>>>>> directly? Is there a
> >>>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
> >>>>>>>>>>>>> differentiation (legacy v.s. modern) needed?
> >>>>>>>>>>>> BTW a good API could be
> >>>>>>>>>>>>
> >>>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>>>>>
> >>>>>>>>>>>> we did it per vring but maybe that was a mistake ...
> >>>>>>>>>>> Actually, I wonder whether it's good time to just not support
> >>>>>>>>>>> legacy driver
> >>>>>>>>>>> for vDPA. Consider:
> >>>>>>>>>>>
> >>>>>>>>>>> 1) It's definition is no-normative
> >>>>>>>>>>> 2) A lot of budren of codes
> >>>>>>>>>>>
> >>>>>>>>>>> So qemu can still present the legacy device since the config
> >>>>>>>>>>> space or other
> >>>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
> >>>>>>>>>>> accessed by
> >>>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
> >>>>>>>>>>> in this
> >>>>>>>>>>> case?
> >>>>>>>>>>>
> >>>>>>>>>>> Thanks
> >>>>>>>>>>>
> >>>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
> >>>>>>>>>> working userspace, qemu releases with vdpa support are out there and
> >>>>>>>>>> seem to work for people. Any changes need to take that into account
> >>>>>>>>>> and document compatibility concerns.
> >>>>>>>>> Agree, let me check.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>>       I note that any hardware
> >>>>>>>>>> implementation is already broken for legacy except on platforms with
> >>>>>>>>>> strong ordering which might be helpful in reducing the scope.
> >>>>>>>>> Yes.
> >>>>>>>>>
> >>>>>>>>> Thanks
> >>>>>>>>>
> >>>>>>>>>
>

Jason Wang Dec. 16, 2021, 3:43 a.m. UTC | #80

On Thu, Dec 16, 2021 at 4:52 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 12/14/2021 6:06 PM, Jason Wang wrote:
> > On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
> >>
> >>
> >> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> >>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> >>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> >>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> >>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> >>>>>> it ended up with. I have the following questions,
> >>>>>>
> >>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
> >>>>>> support will be completely dropped from the table, is my understanding
> >>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> >>>>>> which is backed by the spec at
> >>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> >>>>>> if there's request/need to support wilder legacy virtio versions earlier
> >>>>>> beyond.
> >>>>> I personally feel it's less work to add in kernel than try to
> >>>>> work around it in userspace. Jason feels differently.
> >>>>> Maybe post the patches and this will prove to Jason it's not
> >>>>> too terrible?
> >>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> >>>> layout level and is limited to x86 only, there should be easy way out.
> >>> Note a subtle difference: what matters is that guest, not host is x86.
> >>> Matters for emulators which might reorder memory accesses.
> >>> I guess this enforcement belongs in QEMU then?
> >> Right, I mean to get started, the initial guest driver support and the
> >> corresponding QEMU support for transitional vdpa backend can be limited
> >> to x86 guest/host only. Since the config space is emulated in QEMU, I
> >> suppose it's not hard to enforce in QEMU.
> > It's more than just config space, most devices have headers before the buffer.
> The ordering in datapath (data VQs) would have to rely on vendor's
> support. Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w
> vendor nowadays can/should well support the case when ORDER_PLATFORM is
> not acked by the driver (actually this feature is filtered out by the
> QEMU vhost-vdpa driver today), even with v1.0 spec conforming and modern
> only vDPA device.

That's a bug that needs to be fixed.

> The control VQ is implemented in software in the
> kernel, which can be easily accommodated/fixed when needed.
>
> >
> >> QEMU can drive GET_LEGACY,
> >> GET_ENDIAN et al ioctls in advance to get the capability from the
> >> individual vendor driver. For that, we need another negotiation protocol
> >> similar to vhost_user's protocol_features between the vdpa kernel and
> >> QEMU, way before the guest driver is ever probed and its feature
> >> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> >> from the device, but we can assume weak ordering for legacy at this
> >> point (x86 only)?
> > I'm lost here, we have get_features() so:
> I assume here you refer to get_device_features() that Eli just changed
> the name.
> >
> > 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> > 2) ORDER_PLATFORM means device requires platform ordering
> >
> > Any reason for having a new API for this?
> Are you going to enforce all vDPA hardware vendors to support the
> transitional model for legacy guest? meaning guest not acknowledging
> VERSION_1 would use the legacy interfaces captured in the spec section
> 7.4 (regarding ring layout, native endianness, message framing, vq
> alignment of 4096, 32bit feature, no features_ok bit in status, IO port
> interface i.e. all the things) instead? Noted we don't yet have a
> set_device_features() that allows the vdpa device to tell whether it is
> operating in transitional or modern-only mode. For software virtio, all
> support for the legacy part in a transitional model has been built up
> there already, however, it's not easy for vDPA vendors to implement all
> the requirements for an all-or-nothing legacy guest support (big endian
> guest for example). To these vendors, the legacy support within a
> transitional model is more of feature to them and it's best to leave
> some flexibility for them to implement partial support for legacy. That
> in turn calls out the need for a vhost-user protocol feature like
> negotiation API that can prohibit those unsupported guest setups to as
> early as backend_init before launching the VM.
>
>
> >
> >>>> I
> >>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> >>>> 0.95 support, it seems all the ingredient had been there already dated back
> >>>> to the DPDK days. The only major thing limiting is in the vDPA software that
> >>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> >>>> a few DMA setup ops, which is virtio 1.0 only.
> >>>>
> >>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
> >>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
> >>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
> >>>>>> of vdpa_get_config() to vdpa_set_config().
> >>>>>>
> >>>>>>            /*
> >>>>>>             * Config accesses aren't supposed to trigger before features are
> >>>>>> set.
> >>>>>>             * If it does happen we assume a legacy guest.
> >>>>>>             */
> >>>>>>            if (!vdev->features_valid)
> >>>>>>                    vdpa_set_features(vdev, 0);
> >>>>>>            ops->get_config(vdev, offset, buf, len);
> >>>>>>
> >>>>>> I can post a patch to fix 2) if there's consensus already reached.
> >>>>>>
> >>>>>> Thanks,
> >>>>>> -Siwei
> >>>>> I'm not sure how important it is to change that.
> >>>>> In any case it only affects transitional devices, right?
> >>>>> Legacy only should not care ...
> >>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> >>>> modern one in a transitional device model rather than being legacy only.
> >>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> >>>> guests without having to reconfigure. Or are you suggesting limit to legacy
> >>>> only at the time of vdpa creation would simplify the implementation a lot?
> >>>>
> >>>> Thanks,
> >>>> -Siwei
> >>> I don't know for sure. Take a look at the work Halil was doing
> >>> to try and support transitional devices with BE guests.
> >> Hmmm, we can have those endianness ioctls defined but the initial QEMU
> >> implementation can be started to support x86 guest/host with little
> >> endian and weak memory ordering first. The real trick is to detect
> >> legacy guest - I am not sure if it's feasible to shift all the legacy
> >> detection work to QEMU, or the kernel has to be part of the detection
> >> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> >> effort in QEMU) as well. Let me take a further look and get back.
> > Michael may think differently but I think doing this in Qemu is much easier.
> I think the key is whether we position emulating legacy interfaces in
> QEMU doing translation on top of a v1.0 modern-only device in the
> kernel, or we allow vdpa core (or you can say vhost-vdpa) and vendor
> driver to support a transitional model in the kernel that is able to
> work for both v0.95 and v1.0 drivers, with some slight aid from QEMU for
> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for
> the former we still rely on vendor for a performant data vqs
> implementation, leaving the question to what it may end up eventually in
> the kernel is effectively the latter).

I think we can do the legacy interface emulation on top of the shadow
VQ. And we know it works for sure. But I agree, it would be much
easier if we depend on the vendor to implement a transitional device.

So assuming we depend on the vendor, I don't see anything that is
strictly needed in the kernel, the kick or config access before
DRIVER_OK can all be handled easily in Qemu unless I miss something.
The only value to do that in the kernel is that it can work for
virtio-vdpa, but modern only virito-vpda is sufficient; we don't need
any legacy stuff for that.

Thanks

>
> Thanks,
> -Siwei
>
> >
> > Thanks
> >
> >
> >
> >> Meanwhile, I'll check internally to see if a legacy only model would
> >> work. Thanks.
> >>
> >> Thanks,
> >> -Siwei
> >>
> >>
> >>>
> >>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
> >>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> >>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> >>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> >>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> >>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
> >>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
> >>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
> >>>>>>>>>>> know what the use
> >>>>>>>>>>> case there will be for kernel to leverage such info
> >>>>>>>>>>> directly? Is there a
> >>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
> >>>>>>>>>>> differentiation (legacy v.s. modern) needed?
> >>>>>>>>>> BTW a good API could be
> >>>>>>>>>>
> >>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>>>
> >>>>>>>>>> we did it per vring but maybe that was a mistake ...
> >>>>>>>>> Actually, I wonder whether it's good time to just not support
> >>>>>>>>> legacy driver
> >>>>>>>>> for vDPA. Consider:
> >>>>>>>>>
> >>>>>>>>> 1) It's definition is no-normative
> >>>>>>>>> 2) A lot of budren of codes
> >>>>>>>>>
> >>>>>>>>> So qemu can still present the legacy device since the config
> >>>>>>>>> space or other
> >>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
> >>>>>>>>> accessed by
> >>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
> >>>>>>>>> in this
> >>>>>>>>> case?
> >>>>>>>>>
> >>>>>>>>> Thanks
> >>>>>>>>>
> >>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
> >>>>>>>> working userspace, qemu releases with vdpa support are out there and
> >>>>>>>> seem to work for people. Any changes need to take that into account
> >>>>>>>> and document compatibility concerns.
> >>>>>>> Agree, let me check.
> >>>>>>>
> >>>>>>>
> >>>>>>>>      I note that any hardware
> >>>>>>>> implementation is already broken for legacy except on platforms with
> >>>>>>>> strong ordering which might be helpful in reducing the scope.
> >>>>>>> Yes.
> >>>>>>>
> >>>>>>> Thanks
> >>>>>>>
> >>>>>>>
>

Michael S. Tsirkin Dec. 16, 2021, 6:35 a.m. UTC | #81

On Wed, Dec 15, 2021 at 06:01:55PM -0800, Si-Wei Liu wrote:
> 
> 
> On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:
> > On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
> > > 
> > > On 12/14/2021 6:06 PM, Jason Wang wrote:
> > > > On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
> > > > > 
> > > > > On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> > > > > > On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> > > > > > > On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> > > > > > > > On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > > > > > > > > Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > > > > > > > > it ended up with. I have the following questions,
> > > > > > > > > 
> > > > > > > > > 1. legacy guest support: from the past conversations it doesn't seem the
> > > > > > > > > support will be completely dropped from the table, is my understanding
> > > > > > > > > correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > > > > > > > > which is backed by the spec at
> > > > > > > > > https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> > > > > > > > > if there's request/need to support wilder legacy virtio versions earlier
> > > > > > > > > beyond.
> > > > > > > > I personally feel it's less work to add in kernel than try to
> > > > > > > > work around it in userspace. Jason feels differently.
> > > > > > > > Maybe post the patches and this will prove to Jason it's not
> > > > > > > > too terrible?
> > > > > > > I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> > > > > > > layout level and is limited to x86 only, there should be easy way out.
> > > > > > Note a subtle difference: what matters is that guest, not host is x86.
> > > > > > Matters for emulators which might reorder memory accesses.
> > > > > > I guess this enforcement belongs in QEMU then?
> > > > > Right, I mean to get started, the initial guest driver support and the
> > > > > corresponding QEMU support for transitional vdpa backend can be limited
> > > > > to x86 guest/host only. Since the config space is emulated in QEMU, I
> > > > > suppose it's not hard to enforce in QEMU.
> > > > It's more than just config space, most devices have headers before the buffer.
> > > The ordering in datapath (data VQs) would have to rely on vendor's support.
> > > Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
> > > can/should well support the case when ORDER_PLATFORM is not acked by the
> > > driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
> > > today), even with v1.0 spec conforming and modern only vDPA device. The
> > > control VQ is implemented in software in the kernel, which can be easily
> > > accommodated/fixed when needed.
> > > 
> > > > > QEMU can drive GET_LEGACY,
> > > > > GET_ENDIAN et al ioctls in advance to get the capability from the
> > > > > individual vendor driver. For that, we need another negotiation protocol
> > > > > similar to vhost_user's protocol_features between the vdpa kernel and
> > > > > QEMU, way before the guest driver is ever probed and its feature
> > > > > negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> > > > > from the device, but we can assume weak ordering for legacy at this
> > > > > point (x86 only)?
> > > > I'm lost here, we have get_features() so:
> > > I assume here you refer to get_device_features() that Eli just changed the
> > > name.
> > > > 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> > > > 2) ORDER_PLATFORM means device requires platform ordering
> > > > 
> > > > Any reason for having a new API for this?
> > > Are you going to enforce all vDPA hardware vendors to support the
> > > transitional model for legacy guest? meaning guest not acknowledging
> > > VERSION_1 would use the legacy interfaces captured in the spec section 7.4
> > > (regarding ring layout, native endianness, message framing, vq alignment of
> > > 4096, 32bit feature, no features_ok bit in status, IO port interface i.e.
> > > all the things) instead? Noted we don't yet have a set_device_features()
> > > that allows the vdpa device to tell whether it is operating in transitional
> > > or modern-only mode. For software virtio, all support for the legacy part in
> > > a transitional model has been built up there already, however, it's not easy
> > > for vDPA vendors to implement all the requirements for an all-or-nothing
> > > legacy guest support (big endian guest for example). To these vendors, the
> > > legacy support within a transitional model is more of feature to them and
> > > it's best to leave some flexibility for them to implement partial support
> > > for legacy. That in turn calls out the need for a vhost-user protocol
> > > feature like negotiation API that can prohibit those unsupported guest
> > > setups to as early as backend_init before launching the VM.
> > Right. Of note is the fact that it's a spec bug which I
> > hope yet to fix, though due to existing guest code the
> > fix won't be complete.
> I thought at one point you pointed out to me that the spec does allow config
> space read before claiming features_ok, and only config write before
> features_ok is prohibited. I haven't read up the full thread of Halil's
> VERSION_1 for transitional big endian device yet, but what is the spec bug
> you hope to fix?

Allowing config space reads before features_ok seemed useful years ago
but in practice is only causing bugs and complicating device design.

> 
> > 
> > WRT ioctls, One thing we can do though is abuse set_features
> > where it's called by QEMU early on with just the VERSION_1
> > bit set, to distinguish between legacy and modern
> > interface. This before config space accesses and FEATURES_OK.
> > 
> > Halil has been working on this, pls take a look and maybe help him out.
> Interesting thread, am reading now and see how I may leverage or help there.
> 
> > > > > > > I
> > > > > > > checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> > > > > > > 0.95 support, it seems all the ingredient had been there already dated back
> > > > > > > to the DPDK days. The only major thing limiting is in the vDPA software that
> > > > > > > the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> > > > > > > a few DMA setup ops, which is virtio 1.0 only.
> > > > > > > 
> > > > > > > > > 2. suppose some form of legacy guest support needs to be there, how do we
> > > > > > > > > deal with the bogus assumption below in vdpa_get_config() in the short term?
> > > > > > > > > It looks one of the intuitive fix is to move the vdpa_set_features call out
> > > > > > > > > of vdpa_get_config() to vdpa_set_config().
> > > > > > > > > 
> > > > > > > > >             /*
> > > > > > > > >              * Config accesses aren't supposed to trigger before features are
> > > > > > > > > set.
> > > > > > > > >              * If it does happen we assume a legacy guest.
> > > > > > > > >              */
> > > > > > > > >             if (!vdev->features_valid)
> > > > > > > > >                     vdpa_set_features(vdev, 0);
> > > > > > > > >             ops->get_config(vdev, offset, buf, len);
> > > > > > > > > 
> > > > > > > > > I can post a patch to fix 2) if there's consensus already reached.
> > > > > > > > > 
> > > > > > > > > Thanks,
> > > > > > > > > -Siwei
> > > > > > > > I'm not sure how important it is to change that.
> > > > > > > > In any case it only affects transitional devices, right?
> > > > > > > > Legacy only should not care ...
> > > > > > > Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> > > > > > > modern one in a transitional device model rather than being legacy only.
> > > > > > > That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> > > > > > > guests without having to reconfigure. Or are you suggesting limit to legacy
> > > > > > > only at the time of vdpa creation would simplify the implementation a lot?
> > > > > > > 
> > > > > > > Thanks,
> > > > > > > -Siwei
> > > > > > I don't know for sure. Take a look at the work Halil was doing
> > > > > > to try and support transitional devices with BE guests.
> > > > > Hmmm, we can have those endianness ioctls defined but the initial QEMU
> > > > > implementation can be started to support x86 guest/host with little
> > > > > endian and weak memory ordering first. The real trick is to detect
> > > > > legacy guest - I am not sure if it's feasible to shift all the legacy
> > > > > detection work to QEMU, or the kernel has to be part of the detection
> > > > > (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> > > > > effort in QEMU) as well. Let me take a further look and get back.
> > > > Michael may think differently but I think doing this in Qemu is much easier.
> > > I think the key is whether we position emulating legacy interfaces in QEMU
> > > doing translation on top of a v1.0 modern-only device in the kernel, or we
> > > allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a
> > > transitional model in the kernel that is able to work for both v0.95 and
> > > v1.0 drivers, with some slight aid from QEMU for
> > > detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the
> > > former we still rely on vendor for a performant data vqs implementation,
> > > leaving the question to what it may end up eventually in the kernel is
> > > effectively the latter).
> > > 
> > > Thanks,
> > > -Siwei
> > 
> > My suggestion is post the kernel patches, and we can evaluate
> > how much work they are.
> Thanks for the feedback. I will take some read then get back, probably after
> the winter break. Stay tuned.
> 
> Thanks,
> -Siwei
> 
> > 
> > > > Thanks
> > > > 
> > > > 
> > > > 
> > > > > Meanwhile, I'll check internally to see if a legacy only model would
> > > > > work. Thanks.
> > > > > 
> > > > > Thanks,
> > > > > -Siwei
> > > > > 
> > > > > 
> > > > > > > > > On 3/2/2021 2:53 AM, Jason Wang wrote:
> > > > > > > > > > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > > > > > > > > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > > > > > > > > > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > > > > > > > > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > > > > > > > > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > > > > > > > > > > > > the kernel that it's a legacy guest. Ugh :(
> > > > > > > > > > > > > > Well, although I think adding an ioctl is doable, may I
> > > > > > > > > > > > > > know what the use
> > > > > > > > > > > > > > case there will be for kernel to leverage such info
> > > > > > > > > > > > > > directly? Is there a
> > > > > > > > > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed
> > > > > > > > > > > > > > differentiation (legacy v.s. modern) needed?
> > > > > > > > > > > > > BTW a good API could be
> > > > > > > > > > > > > 
> > > > > > > > > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > > > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > > > > > > > > > > > > 
> > > > > > > > > > > > > we did it per vring but maybe that was a mistake ...
> > > > > > > > > > > > Actually, I wonder whether it's good time to just not support
> > > > > > > > > > > > legacy driver
> > > > > > > > > > > > for vDPA. Consider:
> > > > > > > > > > > > 
> > > > > > > > > > > > 1) It's definition is no-normative
> > > > > > > > > > > > 2) A lot of budren of codes
> > > > > > > > > > > > 
> > > > > > > > > > > > So qemu can still present the legacy device since the config
> > > > > > > > > > > > space or other
> > > > > > > > > > > > stuffs that is presented by vhost-vDPA is not expected to be
> > > > > > > > > > > > accessed by
> > > > > > > > > > > > guest directly. Qemu can do the endian conversion when necessary
> > > > > > > > > > > > in this
> > > > > > > > > > > > case?
> > > > > > > > > > > > 
> > > > > > > > > > > > Thanks
> > > > > > > > > > > > 
> > > > > > > > > > > Overall I would be fine with this approach but we need to avoid breaking
> > > > > > > > > > > working userspace, qemu releases with vdpa support are out there and
> > > > > > > > > > > seem to work for people. Any changes need to take that into account
> > > > > > > > > > > and document compatibility concerns.
> > > > > > > > > > Agree, let me check.
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > >       I note that any hardware
> > > > > > > > > > > implementation is already broken for legacy except on platforms with
> > > > > > > > > > > strong ordering which might be helpful in reducing the scope.
> > > > > > > > > > Yes.
> > > > > > > > > > 
> > > > > > > > > > Thanks
> > > > > > > > > > 
> > > > > > > > > >

Si-Wei Liu Dec. 16, 2021, 10:32 p.m. UTC | #82

On 12/15/2021 6:53 PM, Jason Wang wrote:
> On Thu, Dec 16, 2021 at 10:02 AM Si-Wei Liu <[e-mail]> wrote:
>>
>>
>> On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:
>>> On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
>>>> On 12/14/2021 6:06 PM, Jason Wang wrote:
>>>>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
>>>>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
>>>>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
>>>>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
>>>>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
>>>>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
>>>>>>>>>> it ended up with. I have the following questions,
>>>>>>>>>>
>>>>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
>>>>>>>>>> support will be completely dropped from the table, is my understanding
>>>>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
>>>>>>>>>> which is backed by the spec at
>>>>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
>>>>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
>>>>>>>>>> beyond.
>>>>>>>>> I personally feel it's less work to add in kernel than try to
>>>>>>>>> work around it in userspace. Jason feels differently.
>>>>>>>>> Maybe post the patches and this will prove to Jason it's not
>>>>>>>>> too terrible?
>>>>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
>>>>>>>> layout level and is limited to x86 only, there should be easy way out.
>>>>>>> Note a subtle difference: what matters is that guest, not host is x86.
>>>>>>> Matters for emulators which might reorder memory accesses.
>>>>>>> I guess this enforcement belongs in QEMU then?
>>>>>> Right, I mean to get started, the initial guest driver support and the
>>>>>> corresponding QEMU support for transitional vdpa backend can be limited
>>>>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
>>>>>> suppose it's not hard to enforce in QEMU.
>>>>> It's more than just config space, most devices have headers before the buffer.
>>>> The ordering in datapath (data VQs) would have to rely on vendor's support.
>>>> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
>>>> can/should well support the case when ORDER_PLATFORM is not acked by the
>>>> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
>>>> today), even with v1.0 spec conforming and modern only vDPA device. The
>>>> control VQ is implemented in software in the kernel, which can be easily
>>>> accommodated/fixed when needed.
>>>>
>>>>>> QEMU can drive GET_LEGACY,
>>>>>> GET_ENDIAN et al ioctls in advance to get the capability from the
>>>>>> individual vendor driver. For that, we need another negotiation protocol
>>>>>> similar to vhost_user's protocol_features between the vdpa kernel and
>>>>>> QEMU, way before the guest driver is ever probed and its feature
>>>>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
>>>>>> from the device, but we can assume weak ordering for legacy at this
>>>>>> point (x86 only)?
>>>>> I'm lost here, we have get_features() so:
>>>> I assume here you refer to get_device_features() that Eli just changed the
>>>> name.
>>>>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
>>>>> 2) ORDER_PLATFORM means device requires platform ordering
>>>>>
>>>>> Any reason for having a new API for this?
>>>> Are you going to enforce all vDPA hardware vendors to support the
>>>> transitional model for legacy guest?
> Do we really have other choices?
>
> I suspect the legacy device is never implemented by any vendor:
>
> 1) no virtio way to detect host endian
This is even true for transitional device that is conforming to the 
spec, right? The transport specific way to detect host endian is still 
being discussed and the spec revision is not finalized yet so far as I 
see. Why this suddenly becomes a requirement/blocker for h/w vendors to 
implement the transitional model? Even if the spec is out, this is 
pretty new and I suspect not all vendor would follow right away. I hope 
the software framework can be tolerant with h/w vendors not supporting 
host endianess (BE specifically) or not detecting it if they would like 
to support a transitional device for legacy.

> 2) bypass IOMMU with translated requests
> 3) PIO port
>
> Yes we have enp_vdpa, but it's more like a "transitional device" for
> legacy only guests.
>
>> meaning guest not acknowledging
>>>> VERSION_1 would use the legacy interfaces captured in the spec section 7.4
>>>> (regarding ring layout, native endianness, message framing, vq alignment of
>>>> 4096, 32bit feature, no features_ok bit in status, IO port interface i.e.
>>>> all the things) instead?
> Note that we only care about the datapath, control path is mediated anyhow.
>
> So feature_ok and IO port isn't an issue. The rest looks like a must
> for the hardware.
H/W vendors can opt out not implementing transitional interfaces at all 
which limits itself a modern only device. Set endianess detection (via 
transport specific means) aside, for vendors that wishes to support 
transitional device with legacy interface, is it a hard stop to drop 
supporting BE host if everything else is there? The spec today doesn't 
define virtio specific means to detect host memory ordering or device 
memory coherency, will it yet become a stopper another day for h/w 
vendor to support more platforms?

>
>> Noted we don't yet have a set_device_features()
>>>> that allows the vdpa device to tell whether it is operating in transitional
>>>> or modern-only mode.
> So the device feature should be provisioned via the netlink protocol.
Such netlink interface will only be used to limit feature exposure, 
right? i.e. you can limit a transitional supporting vendor driver to 
offering modern-only interface, but you never want to make a modern-only 
vendor driver to support transitional (I'm not sure if it's a good idea 
to support all the translation in software, esp. for datapath).
> And what we want is not "set_device_feature()" but
> "set_device_mandatory_feautre()", then the parent can choose to fail
> the negotiation when VERSION_1 is not negotiated.
This assumes the transport specific detection of BE host is in place, 
right? I am not clear who initiates the set_device_mandatory_feautre() 
call, QEMU during guest feature negotiation, or admin user setting it 
ahead via netlink?

Thanks,
-Siwei

>   Qemu then knows for
> sure it talks to a transitional device or modern only device.
>
> Thanks
>
>> For software virtio, all support for the legacy part in
>>>> a transitional model has been built up there already, however, it's not easy
>>>> for vDPA vendors to implement all the requirements for an all-or-nothing
>>>> legacy guest support (big endian guest for example). To these vendors, the
>>>> legacy support within a transitional model is more of feature to them and
>>>> it's best to leave some flexibility for them to implement partial support
>>>> for legacy. That in turn calls out the need for a vhost-user protocol
>>>> feature like negotiation API that can prohibit those unsupported guest
>>>> setups to as early as backend_init before launching the VM.
>>> Right. Of note is the fact that it's a spec bug which I
>>> hope yet to fix, though due to existing guest code the
>>> fix won't be complete.
>> I thought at one point you pointed out to me that the spec does allow
>> config space read before claiming features_ok, and only config write
>> before features_ok is prohibited. I haven't read up the full thread of
>> Halil's VERSION_1 for transitional big endian device yet, but what is
>> the spec bug you hope to fix?
>>
>>> WRT ioctls, One thing we can do though is abuse set_features
>>> where it's called by QEMU early on with just the VERSION_1
>>> bit set, to distinguish between legacy and modern
>>> interface. This before config space accesses and FEATURES_OK.
>>>
>>> Halil has been working on this, pls take a look and maybe help him out.
>> Interesting thread, am reading now and see how I may leverage or help there.
>>
>>>>>>>> I
>>>>>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
>>>>>>>> 0.95 support, it seems all the ingredient had been there already dated back
>>>>>>>> to the DPDK days. The only major thing limiting is in the vDPA software that
>>>>>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
>>>>>>>> a few DMA setup ops, which is virtio 1.0 only.
>>>>>>>>
>>>>>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
>>>>>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
>>>>>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
>>>>>>>>>> of vdpa_get_config() to vdpa_set_config().
>>>>>>>>>>
>>>>>>>>>>              /*
>>>>>>>>>>               * Config accesses aren't supposed to trigger before features are
>>>>>>>>>> set.
>>>>>>>>>>               * If it does happen we assume a legacy guest.
>>>>>>>>>>               */
>>>>>>>>>>              if (!vdev->features_valid)
>>>>>>>>>>                      vdpa_set_features(vdev, 0);
>>>>>>>>>>              ops->get_config(vdev, offset, buf, len);
>>>>>>>>>>
>>>>>>>>>> I can post a patch to fix 2) if there's consensus already reached.
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>> -Siwei
>>>>>>>>> I'm not sure how important it is to change that.
>>>>>>>>> In any case it only affects transitional devices, right?
>>>>>>>>> Legacy only should not care ...
>>>>>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
>>>>>>>> modern one in a transitional device model rather than being legacy only.
>>>>>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
>>>>>>>> guests without having to reconfigure. Or are you suggesting limit to legacy
>>>>>>>> only at the time of vdpa creation would simplify the implementation a lot?
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> -Siwei
>>>>>>> I don't know for sure. Take a look at the work Halil was doing
>>>>>>> to try and support transitional devices with BE guests.
>>>>>> Hmmm, we can have those endianness ioctls defined but the initial QEMU
>>>>>> implementation can be started to support x86 guest/host with little
>>>>>> endian and weak memory ordering first. The real trick is to detect
>>>>>> legacy guest - I am not sure if it's feasible to shift all the legacy
>>>>>> detection work to QEMU, or the kernel has to be part of the detection
>>>>>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
>>>>>> effort in QEMU) as well. Let me take a further look and get back.
>>>>> Michael may think differently but I think doing this in Qemu is much easier.
>>>> I think the key is whether we position emulating legacy interfaces in QEMU
>>>> doing translation on top of a v1.0 modern-only device in the kernel, or we
>>>> allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a
>>>> transitional model in the kernel that is able to work for both v0.95 and
>>>> v1.0 drivers, with some slight aid from QEMU for
>>>> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the
>>>> former we still rely on vendor for a performant data vqs implementation,
>>>> leaving the question to what it may end up eventually in the kernel is
>>>> effectively the latter).
>>>>
>>>> Thanks,
>>>> -Siwei
>>> My suggestion is post the kernel patches, and we can evaluate
>>> how much work they are.
>> Thanks for the feedback. I will take some read then get back, probably
>> after the winter break. Stay tuned.
>>
>> Thanks,
>> -Siwei
>>
>>>>> Thanks
>>>>>
>>>>>
>>>>>
>>>>>> Meanwhile, I'll check internally to see if a legacy only model would
>>>>>> work. Thanks.
>>>>>>
>>>>>> Thanks,
>>>>>> -Siwei
>>>>>>
>>>>>>
>>>>>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
>>>>>>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
>>>>>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>>>>>>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>>>>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
>>>>>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
>>>>>>>>>>>>>>> know what the use
>>>>>>>>>>>>>>> case there will be for kernel to leverage such info
>>>>>>>>>>>>>>> directly? Is there a
>>>>>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>>>>>>>>>>>>> differentiation (legacy v.s. modern) needed?
>>>>>>>>>>>>>> BTW a good API could be
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> we did it per vring but maybe that was a mistake ...
>>>>>>>>>>>>> Actually, I wonder whether it's good time to just not support
>>>>>>>>>>>>> legacy driver
>>>>>>>>>>>>> for vDPA. Consider:
>>>>>>>>>>>>>
>>>>>>>>>>>>> 1) It's definition is no-normative
>>>>>>>>>>>>> 2) A lot of budren of codes
>>>>>>>>>>>>>
>>>>>>>>>>>>> So qemu can still present the legacy device since the config
>>>>>>>>>>>>> space or other
>>>>>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
>>>>>>>>>>>>> accessed by
>>>>>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
>>>>>>>>>>>>> in this
>>>>>>>>>>>>> case?
>>>>>>>>>>>>>
>>>>>>>>>>>>> Thanks
>>>>>>>>>>>>>
>>>>>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
>>>>>>>>>>>> working userspace, qemu releases with vdpa support are out there and
>>>>>>>>>>>> seem to work for people. Any changes need to take that into account
>>>>>>>>>>>> and document compatibility concerns.
>>>>>>>>>>> Agree, let me check.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>>        I note that any hardware
>>>>>>>>>>>> implementation is already broken for legacy except on platforms with
>>>>>>>>>>>> strong ordering which might be helpful in reducing the scope.
>>>>>>>>>>> Yes.
>>>>>>>>>>>
>>>>>>>>>>> Thanks
>>>>>>>>>>>
>>>>>>>>>>>

Si-Wei Liu Dec. 17, 2021, 1:08 a.m. UTC | #83

On 12/15/2021 7:43 PM, Jason Wang wrote:
> On Thu, Dec 16, 2021 at 4:52 AM Si-Wei Liu <[e-mail]> wrote:
>>
>>
>> On 12/14/2021 6:06 PM, Jason Wang wrote:
>>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
>>>>
>>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
>>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
>>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
>>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
>>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
>>>>>>>> it ended up with. I have the following questions,
>>>>>>>>
>>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
>>>>>>>> support will be completely dropped from the table, is my understanding
>>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
>>>>>>>> which is backed by the spec at
>>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
>>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
>>>>>>>> beyond.
>>>>>>> I personally feel it's less work to add in kernel than try to
>>>>>>> work around it in userspace. Jason feels differently.
>>>>>>> Maybe post the patches and this will prove to Jason it's not
>>>>>>> too terrible?
>>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
>>>>>> layout level and is limited to x86 only, there should be easy way out.
>>>>> Note a subtle difference: what matters is that guest, not host is x86.
>>>>> Matters for emulators which might reorder memory accesses.
>>>>> I guess this enforcement belongs in QEMU then?
>>>> Right, I mean to get started, the initial guest driver support and the
>>>> corresponding QEMU support for transitional vdpa backend can be limited
>>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
>>>> suppose it's not hard to enforce in QEMU.
>>> It's more than just config space, most devices have headers before the buffer.
>> The ordering in datapath (data VQs) would have to rely on vendor's
>> support. Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w
>> vendor nowadays can/should well support the case when ORDER_PLATFORM is
>> not acked by the driver (actually this feature is filtered out by the
>> QEMU vhost-vdpa driver today), even with v1.0 spec conforming and modern
>> only vDPA device.
> That's a bug that needs to be fixed.
>
>> The control VQ is implemented in software in the
>> kernel, which can be easily accommodated/fixed when needed.
>>
>>>> QEMU can drive GET_LEGACY,
>>>> GET_ENDIAN et al ioctls in advance to get the capability from the
>>>> individual vendor driver. For that, we need another negotiation protocol
>>>> similar to vhost_user's protocol_features between the vdpa kernel and
>>>> QEMU, way before the guest driver is ever probed and its feature
>>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
>>>> from the device, but we can assume weak ordering for legacy at this
>>>> point (x86 only)?
>>> I'm lost here, we have get_features() so:
>> I assume here you refer to get_device_features() that Eli just changed
>> the name.
>>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
>>> 2) ORDER_PLATFORM means device requires platform ordering
>>>
>>> Any reason for having a new API for this?
>> Are you going to enforce all vDPA hardware vendors to support the
>> transitional model for legacy guest? meaning guest not acknowledging
>> VERSION_1 would use the legacy interfaces captured in the spec section
>> 7.4 (regarding ring layout, native endianness, message framing, vq
>> alignment of 4096, 32bit feature, no features_ok bit in status, IO port
>> interface i.e. all the things) instead? Noted we don't yet have a
>> set_device_features() that allows the vdpa device to tell whether it is
>> operating in transitional or modern-only mode. For software virtio, all
>> support for the legacy part in a transitional model has been built up
>> there already, however, it's not easy for vDPA vendors to implement all
>> the requirements for an all-or-nothing legacy guest support (big endian
>> guest for example). To these vendors, the legacy support within a
>> transitional model is more of feature to them and it's best to leave
>> some flexibility for them to implement partial support for legacy. That
>> in turn calls out the need for a vhost-user protocol feature like
>> negotiation API that can prohibit those unsupported guest setups to as
>> early as backend_init before launching the VM.
>>
>>
>>>>>> I
>>>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
>>>>>> 0.95 support, it seems all the ingredient had been there already dated back
>>>>>> to the DPDK days. The only major thing limiting is in the vDPA software that
>>>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
>>>>>> a few DMA setup ops, which is virtio 1.0 only.
>>>>>>
>>>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
>>>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
>>>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
>>>>>>>> of vdpa_get_config() to vdpa_set_config().
>>>>>>>>
>>>>>>>>             /*
>>>>>>>>              * Config accesses aren't supposed to trigger before features are
>>>>>>>> set.
>>>>>>>>              * If it does happen we assume a legacy guest.
>>>>>>>>              */
>>>>>>>>             if (!vdev->features_valid)
>>>>>>>>                     vdpa_set_features(vdev, 0);
>>>>>>>>             ops->get_config(vdev, offset, buf, len);
>>>>>>>>
>>>>>>>> I can post a patch to fix 2) if there's consensus already reached.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> -Siwei
>>>>>>> I'm not sure how important it is to change that.
>>>>>>> In any case it only affects transitional devices, right?
>>>>>>> Legacy only should not care ...
>>>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
>>>>>> modern one in a transitional device model rather than being legacy only.
>>>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
>>>>>> guests without having to reconfigure. Or are you suggesting limit to legacy
>>>>>> only at the time of vdpa creation would simplify the implementation a lot?
>>>>>>
>>>>>> Thanks,
>>>>>> -Siwei
>>>>> I don't know for sure. Take a look at the work Halil was doing
>>>>> to try and support transitional devices with BE guests.
>>>> Hmmm, we can have those endianness ioctls defined but the initial QEMU
>>>> implementation can be started to support x86 guest/host with little
>>>> endian and weak memory ordering first. The real trick is to detect
>>>> legacy guest - I am not sure if it's feasible to shift all the legacy
>>>> detection work to QEMU, or the kernel has to be part of the detection
>>>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
>>>> effort in QEMU) as well. Let me take a further look and get back.
>>> Michael may think differently but I think doing this in Qemu is much easier.
>> I think the key is whether we position emulating legacy interfaces in
>> QEMU doing translation on top of a v1.0 modern-only device in the
>> kernel, or we allow vdpa core (or you can say vhost-vdpa) and vendor
>> driver to support a transitional model in the kernel that is able to
>> work for both v0.95 and v1.0 drivers, with some slight aid from QEMU for
>> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for
>> the former we still rely on vendor for a performant data vqs
>> implementation, leaving the question to what it may end up eventually in
>> the kernel is effectively the latter).
> I think we can do the legacy interface emulation on top of the shadow
> VQ. And we know it works for sure. But I agree, it would be much
> easier if we depend on the vendor to implement a transitional device.
First I am not sure if there's a convincing case for users to deploy 
vDPA with shadow (data) VQ against the pure software based backend. 
Please enlighten me if there is.

For us, the point to deploy vDPA for legacy guest is the acceleration 
(what "A" stands for in "vDPA") part of it so that we can leverage the 
hardware potential if at all possible. Not sure how the shadow VQ 
implementation can easily deal with datapath acceleration without losing 
too much performance?

> So assuming we depend on the vendor, I don't see anything that is
> strictly needed in the kernel, the kick or config access before
> DRIVER_OK can all be handled easily in Qemu unless I miss something.
Right, that's what I think too it's not quite a lot of work in the 
kernel if vendor device offers the aid/support for transitional. The 
kernel only provides the abstraction of device model (transitional or 
modern-only), while vendor driver may implement early platform feature 
discovery and apply legacy specific quirks (unsupported endianness, 
mismatched page size, unsupported host memory ordering model) that the 
device can't adapt to. I don't say we have to depend on the vendor, but 
the point is that we must assume fully spec compliant transitional 
support (the datapath in particular) from the vendor to get started, as 
I guess it's probably the main motivation for users to deploy it - 
acceleration of legacy guest workload without exhausting host computing 
resource. Even if we get started with shadow VQ to mediate and translate 
the datapath, eventually it may evolve towards leveraging datapath 
offload to hardware if acceleration is the only convincing use case for 
legacy support.

Thanks,
-Siwei
> The only value to do that in the kernel is that it can work for
> virtio-vdpa, but modern only virito-vpda is sufficient; we don't need
> any legacy stuff for that.
>
> Thanks
>
>> Thanks,
>> -Siwei
>>
>>> Thanks
>>>
>>>
>>>
>>>> Meanwhile, I'll check internally to see if a legacy only model would
>>>> work. Thanks.
>>>>
>>>> Thanks,
>>>> -Siwei
>>>>
>>>>
>>>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
>>>>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
>>>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
>>>>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
>>>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
>>>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
>>>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
>>>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
>>>>>>>>>>>>> know what the use
>>>>>>>>>>>>> case there will be for kernel to leverage such info
>>>>>>>>>>>>> directly? Is there a
>>>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
>>>>>>>>>>>>> differentiation (legacy v.s. modern) needed?
>>>>>>>>>>>> BTW a good API could be
>>>>>>>>>>>>
>>>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
>>>>>>>>>>>>
>>>>>>>>>>>> we did it per vring but maybe that was a mistake ...
>>>>>>>>>>> Actually, I wonder whether it's good time to just not support
>>>>>>>>>>> legacy driver
>>>>>>>>>>> for vDPA. Consider:
>>>>>>>>>>>
>>>>>>>>>>> 1) It's definition is no-normative
>>>>>>>>>>> 2) A lot of budren of codes
>>>>>>>>>>>
>>>>>>>>>>> So qemu can still present the legacy device since the config
>>>>>>>>>>> space or other
>>>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
>>>>>>>>>>> accessed by
>>>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
>>>>>>>>>>> in this
>>>>>>>>>>> case?
>>>>>>>>>>>
>>>>>>>>>>> Thanks
>>>>>>>>>>>
>>>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
>>>>>>>>>> working userspace, qemu releases with vdpa support are out there and
>>>>>>>>>> seem to work for people. Any changes need to take that into account
>>>>>>>>>> and document compatibility concerns.
>>>>>>>>> Agree, let me check.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>       I note that any hardware
>>>>>>>>>> implementation is already broken for legacy except on platforms with
>>>>>>>>>> strong ordering which might be helpful in reducing the scope.
>>>>>>>>> Yes.
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>>
>>>>>>>>>

Jason Wang Dec. 17, 2021, 1:57 a.m. UTC | #84

On Fri, Dec 17, 2021 at 6:32 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 12/15/2021 6:53 PM, Jason Wang wrote:
> > On Thu, Dec 16, 2021 at 10:02 AM Si-Wei Liu <[e-mail]> wrote:
> >>
> >>
> >> On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:
> >>> On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
> >>>> On 12/14/2021 6:06 PM, Jason Wang wrote:
> >>>>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> >>>>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> >>>>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> >>>>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> >>>>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> >>>>>>>>>> it ended up with. I have the following questions,
> >>>>>>>>>>
> >>>>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
> >>>>>>>>>> support will be completely dropped from the table, is my understanding
> >>>>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> >>>>>>>>>> which is backed by the spec at
> >>>>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> >>>>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
> >>>>>>>>>> beyond.
> >>>>>>>>> I personally feel it's less work to add in kernel than try to
> >>>>>>>>> work around it in userspace. Jason feels differently.
> >>>>>>>>> Maybe post the patches and this will prove to Jason it's not
> >>>>>>>>> too terrible?
> >>>>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> >>>>>>>> layout level and is limited to x86 only, there should be easy way out.
> >>>>>>> Note a subtle difference: what matters is that guest, not host is x86.
> >>>>>>> Matters for emulators which might reorder memory accesses.
> >>>>>>> I guess this enforcement belongs in QEMU then?
> >>>>>> Right, I mean to get started, the initial guest driver support and the
> >>>>>> corresponding QEMU support for transitional vdpa backend can be limited
> >>>>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
> >>>>>> suppose it's not hard to enforce in QEMU.
> >>>>> It's more than just config space, most devices have headers before the buffer.
> >>>> The ordering in datapath (data VQs) would have to rely on vendor's support.
> >>>> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
> >>>> can/should well support the case when ORDER_PLATFORM is not acked by the
> >>>> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
> >>>> today), even with v1.0 spec conforming and modern only vDPA device. The
> >>>> control VQ is implemented in software in the kernel, which can be easily
> >>>> accommodated/fixed when needed.
> >>>>
> >>>>>> QEMU can drive GET_LEGACY,
> >>>>>> GET_ENDIAN et al ioctls in advance to get the capability from the
> >>>>>> individual vendor driver. For that, we need another negotiation protocol
> >>>>>> similar to vhost_user's protocol_features between the vdpa kernel and
> >>>>>> QEMU, way before the guest driver is ever probed and its feature
> >>>>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> >>>>>> from the device, but we can assume weak ordering for legacy at this
> >>>>>> point (x86 only)?
> >>>>> I'm lost here, we have get_features() so:
> >>>> I assume here you refer to get_device_features() that Eli just changed the
> >>>> name.
> >>>>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> >>>>> 2) ORDER_PLATFORM means device requires platform ordering
> >>>>>
> >>>>> Any reason for having a new API for this?
> >>>> Are you going to enforce all vDPA hardware vendors to support the
> >>>> transitional model for legacy guest?
> > Do we really have other choices?
> >
> > I suspect the legacy device is never implemented by any vendor:
> >
> > 1) no virtio way to detect host endian
> This is even true for transitional device that is conforming to the
> spec, right?

For hardware, yes.

> The transport specific way to detect host endian is still
> being discussed and the spec revision is not finalized yet so far as I
> see. Why this suddenly becomes a requirement/blocker for h/w vendors to
> implement the transitional model?

It's not a sudden blocker, the problem has existed since day 0 if I
was not wrong. That's why the problem looks a little bit complicated
and why it would be much simpler if we stick to modern devices.

> Even if the spec is out, this is
> pretty new and I suspect not all vendor would follow right away. I hope
> the software framework can be tolerant with h/w vendors not supporting
> host endianess (BE specifically) or not detecting it if they would like
> to support a transitional device for legacy.

Well, if we know we don't want to support the BE host it would be fine.

>
> > 2) bypass IOMMU with translated requests
> > 3) PIO port
> >
> > Yes we have enp_vdpa, but it's more like a "transitional device" for
> > legacy only guests.
> >
> >> meaning guest not acknowledging
> >>>> VERSION_1 would use the legacy interfaces captured in the spec section 7.4
> >>>> (regarding ring layout, native endianness, message framing, vq alignment of
> >>>> 4096, 32bit feature, no features_ok bit in status, IO port interface i.e.
> >>>> all the things) instead?
> > Note that we only care about the datapath, control path is mediated anyhow.
> >
> > So feature_ok and IO port isn't an issue. The rest looks like a must
> > for the hardware.
> H/W vendors can opt out not implementing transitional interfaces at all
> which limits itself a modern only device. Set endianess detection (via
> transport specific means) aside, for vendors that wishes to support
> transitional device with legacy interface, is it a hard stop to drop
> supporting BE host if everything else is there? The spec today doesn't
> define virtio specific means to detect host memory ordering or device
> memory coherency,

Any reason that we need to care about memory coherency at the virtio
level. I'd expect it's the task of transport.

> will it yet become a stopper another day for h/w
> vendor to support more platforms?

Let's differentiate virtio from vdpa here. For virtio, there's no way
to add any feature for legacy devices. We can only add memory features
detecting for modern devices.

But for vDPA, we can introduce any API that can help vendors to
present a transitional device. But we can force those APIs since it's
too late to do that. So transitional devices support is optional for
sure.

>
> >
> >> Noted we don't yet have a set_device_features()
> >>>> that allows the vdpa device to tell whether it is operating in transitional
> >>>> or modern-only mode.
> > So the device feature should be provisioned via the netlink protocol.
> Such netlink interface will only be used to limit feature exposure,
> right? i.e. you can limit a transitional supporting vendor driver to
> offering modern-only interface,

There's no way for the management to force a feature, like VERSION_1
via the current protocol.

> but you never want to make a modern-only
> vendor driver to support transitional (I'm not sure if it's a good idea
> to support all the translation in software, esp. for datapath).

You may hit this problem for sure, you can't force all vendors to
support transitional devices especially considering spec said legacy
is optional. We don't want to end up with a userspace code that can
only work for some specific vendors.

> > And what we want is not "set_device_feature()" but
> > "set_device_mandatory_feautre()", then the parent can choose to fail
> > the negotiation when VERSION_1 is not negotiated.
> This assumes the transport specific detection of BE host is in place,
> right?

Again, the point is, we can not assume such detection works for all of
the vendors. And assume BE detection is ready, we still need this for
modern devices, isn't it?

> I am not clear who initiates the set_device_mandatory_feautre()
> call, QEMU during guest feature negotiation, or admin user setting it
> ahead via netlink?

Netlink, actually, the spec needs to be extended as well, we saw
similar requests in the past. E.g there could be a device that works
in a packed layout only.

Thanks

>
> Thanks,
> -Siwei
>
> >   Qemu then knows for
> > sure it talks to a transitional device or modern only device.
> >
> > Thanks
> >
> >> For software virtio, all support for the legacy part in
> >>>> a transitional model has been built up there already, however, it's not easy
> >>>> for vDPA vendors to implement all the requirements for an all-or-nothing
> >>>> legacy guest support (big endian guest for example). To these vendors, the
> >>>> legacy support within a transitional model is more of feature to them and
> >>>> it's best to leave some flexibility for them to implement partial support
> >>>> for legacy. That in turn calls out the need for a vhost-user protocol
> >>>> feature like negotiation API that can prohibit those unsupported guest
> >>>> setups to as early as backend_init before launching the VM.
> >>> Right. Of note is the fact that it's a spec bug which I
> >>> hope yet to fix, though due to existing guest code the
> >>> fix won't be complete.
> >> I thought at one point you pointed out to me that the spec does allow
> >> config space read before claiming features_ok, and only config write
> >> before features_ok is prohibited. I haven't read up the full thread of
> >> Halil's VERSION_1 for transitional big endian device yet, but what is
> >> the spec bug you hope to fix?
> >>
> >>> WRT ioctls, One thing we can do though is abuse set_features
> >>> where it's called by QEMU early on with just the VERSION_1
> >>> bit set, to distinguish between legacy and modern
> >>> interface. This before config space accesses and FEATURES_OK.
> >>>
> >>> Halil has been working on this, pls take a look and maybe help him out.
> >> Interesting thread, am reading now and see how I may leverage or help there.
> >>
> >>>>>>>> I
> >>>>>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> >>>>>>>> 0.95 support, it seems all the ingredient had been there already dated back
> >>>>>>>> to the DPDK days. The only major thing limiting is in the vDPA software that
> >>>>>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> >>>>>>>> a few DMA setup ops, which is virtio 1.0 only.
> >>>>>>>>
> >>>>>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
> >>>>>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
> >>>>>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
> >>>>>>>>>> of vdpa_get_config() to vdpa_set_config().
> >>>>>>>>>>
> >>>>>>>>>>              /*
> >>>>>>>>>>               * Config accesses aren't supposed to trigger before features are
> >>>>>>>>>> set.
> >>>>>>>>>>               * If it does happen we assume a legacy guest.
> >>>>>>>>>>               */
> >>>>>>>>>>              if (!vdev->features_valid)
> >>>>>>>>>>                      vdpa_set_features(vdev, 0);
> >>>>>>>>>>              ops->get_config(vdev, offset, buf, len);
> >>>>>>>>>>
> >>>>>>>>>> I can post a patch to fix 2) if there's consensus already reached.
> >>>>>>>>>>
> >>>>>>>>>> Thanks,
> >>>>>>>>>> -Siwei
> >>>>>>>>> I'm not sure how important it is to change that.
> >>>>>>>>> In any case it only affects transitional devices, right?
> >>>>>>>>> Legacy only should not care ...
> >>>>>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> >>>>>>>> modern one in a transitional device model rather than being legacy only.
> >>>>>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> >>>>>>>> guests without having to reconfigure. Or are you suggesting limit to legacy
> >>>>>>>> only at the time of vdpa creation would simplify the implementation a lot?
> >>>>>>>>
> >>>>>>>> Thanks,
> >>>>>>>> -Siwei
> >>>>>>> I don't know for sure. Take a look at the work Halil was doing
> >>>>>>> to try and support transitional devices with BE guests.
> >>>>>> Hmmm, we can have those endianness ioctls defined but the initial QEMU
> >>>>>> implementation can be started to support x86 guest/host with little
> >>>>>> endian and weak memory ordering first. The real trick is to detect
> >>>>>> legacy guest - I am not sure if it's feasible to shift all the legacy
> >>>>>> detection work to QEMU, or the kernel has to be part of the detection
> >>>>>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> >>>>>> effort in QEMU) as well. Let me take a further look and get back.
> >>>>> Michael may think differently but I think doing this in Qemu is much easier.
> >>>> I think the key is whether we position emulating legacy interfaces in QEMU
> >>>> doing translation on top of a v1.0 modern-only device in the kernel, or we
> >>>> allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a
> >>>> transitional model in the kernel that is able to work for both v0.95 and
> >>>> v1.0 drivers, with some slight aid from QEMU for
> >>>> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the
> >>>> former we still rely on vendor for a performant data vqs implementation,
> >>>> leaving the question to what it may end up eventually in the kernel is
> >>>> effectively the latter).
> >>>>
> >>>> Thanks,
> >>>> -Siwei
> >>> My suggestion is post the kernel patches, and we can evaluate
> >>> how much work they are.
> >> Thanks for the feedback. I will take some read then get back, probably
> >> after the winter break. Stay tuned.
> >>
> >> Thanks,
> >> -Siwei
> >>
> >>>>> Thanks
> >>>>>
> >>>>>
> >>>>>
> >>>>>> Meanwhile, I'll check internally to see if a legacy only model would
> >>>>>> work. Thanks.
> >>>>>>
> >>>>>> Thanks,
> >>>>>> -Siwei
> >>>>>>
> >>>>>>
> >>>>>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
> >>>>>>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> >>>>>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> >>>>>>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> >>>>>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> >>>>>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
> >>>>>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
> >>>>>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
> >>>>>>>>>>>>>>> know what the use
> >>>>>>>>>>>>>>> case there will be for kernel to leverage such info
> >>>>>>>>>>>>>>> directly? Is there a
> >>>>>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
> >>>>>>>>>>>>>>> differentiation (legacy v.s. modern) needed?
> >>>>>>>>>>>>>> BTW a good API could be
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> we did it per vring but maybe that was a mistake ...
> >>>>>>>>>>>>> Actually, I wonder whether it's good time to just not support
> >>>>>>>>>>>>> legacy driver
> >>>>>>>>>>>>> for vDPA. Consider:
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> 1) It's definition is no-normative
> >>>>>>>>>>>>> 2) A lot of budren of codes
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> So qemu can still present the legacy device since the config
> >>>>>>>>>>>>> space or other
> >>>>>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
> >>>>>>>>>>>>> accessed by
> >>>>>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
> >>>>>>>>>>>>> in this
> >>>>>>>>>>>>> case?
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Thanks
> >>>>>>>>>>>>>
> >>>>>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
> >>>>>>>>>>>> working userspace, qemu releases with vdpa support are out there and
> >>>>>>>>>>>> seem to work for people. Any changes need to take that into account
> >>>>>>>>>>>> and document compatibility concerns.
> >>>>>>>>>>> Agree, let me check.
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>>>        I note that any hardware
> >>>>>>>>>>>> implementation is already broken for legacy except on platforms with
> >>>>>>>>>>>> strong ordering which might be helpful in reducing the scope.
> >>>>>>>>>>> Yes.
> >>>>>>>>>>>
> >>>>>>>>>>> Thanks
> >>>>>>>>>>>
> >>>>>>>>>>>
>

Michael S. Tsirkin Dec. 17, 2021, 2 a.m. UTC | #85

On Fri, Dec 17, 2021 at 09:57:38AM +0800, Jason Wang wrote:
> On Fri, Dec 17, 2021 at 6:32 AM Si-Wei Liu <[e-mail]> wrote:
> >
> >
> >
> > On 12/15/2021 6:53 PM, Jason Wang wrote:
> > > On Thu, Dec 16, 2021 at 10:02 AM Si-Wei Liu <[e-mail]> wrote:
> > >>
> > >>
> > >> On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:
> > >>> On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
> > >>>> On 12/14/2021 6:06 PM, Jason Wang wrote:
> > >>>>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
> > >>>>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> > >>>>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> > >>>>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> > >>>>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > >>>>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > >>>>>>>>>> it ended up with. I have the following questions,
> > >>>>>>>>>>
> > >>>>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
> > >>>>>>>>>> support will be completely dropped from the table, is my understanding
> > >>>>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > >>>>>>>>>> which is backed by the spec at
> > >>>>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> > >>>>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
> > >>>>>>>>>> beyond.
> > >>>>>>>>> I personally feel it's less work to add in kernel than try to
> > >>>>>>>>> work around it in userspace. Jason feels differently.
> > >>>>>>>>> Maybe post the patches and this will prove to Jason it's not
> > >>>>>>>>> too terrible?
> > >>>>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> > >>>>>>>> layout level and is limited to x86 only, there should be easy way out.
> > >>>>>>> Note a subtle difference: what matters is that guest, not host is x86.
> > >>>>>>> Matters for emulators which might reorder memory accesses.
> > >>>>>>> I guess this enforcement belongs in QEMU then?
> > >>>>>> Right, I mean to get started, the initial guest driver support and the
> > >>>>>> corresponding QEMU support for transitional vdpa backend can be limited
> > >>>>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
> > >>>>>> suppose it's not hard to enforce in QEMU.
> > >>>>> It's more than just config space, most devices have headers before the buffer.
> > >>>> The ordering in datapath (data VQs) would have to rely on vendor's support.
> > >>>> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
> > >>>> can/should well support the case when ORDER_PLATFORM is not acked by the
> > >>>> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
> > >>>> today), even with v1.0 spec conforming and modern only vDPA device. The
> > >>>> control VQ is implemented in software in the kernel, which can be easily
> > >>>> accommodated/fixed when needed.
> > >>>>
> > >>>>>> QEMU can drive GET_LEGACY,
> > >>>>>> GET_ENDIAN et al ioctls in advance to get the capability from the
> > >>>>>> individual vendor driver. For that, we need another negotiation protocol
> > >>>>>> similar to vhost_user's protocol_features between the vdpa kernel and
> > >>>>>> QEMU, way before the guest driver is ever probed and its feature
> > >>>>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> > >>>>>> from the device, but we can assume weak ordering for legacy at this
> > >>>>>> point (x86 only)?
> > >>>>> I'm lost here, we have get_features() so:
> > >>>> I assume here you refer to get_device_features() that Eli just changed the
> > >>>> name.
> > >>>>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> > >>>>> 2) ORDER_PLATFORM means device requires platform ordering
> > >>>>>
> > >>>>> Any reason for having a new API for this?
> > >>>> Are you going to enforce all vDPA hardware vendors to support the
> > >>>> transitional model for legacy guest?
> > > Do we really have other choices?
> > >
> > > I suspect the legacy device is never implemented by any vendor:
> > >
> > > 1) no virtio way to detect host endian
> > This is even true for transitional device that is conforming to the
> > spec, right?
> 
> For hardware, yes.
> 
> > The transport specific way to detect host endian is still
> > being discussed and the spec revision is not finalized yet so far as I
> > see. Why this suddenly becomes a requirement/blocker for h/w vendors to
> > implement the transitional model?
> 
> It's not a sudden blocker, the problem has existed since day 0 if I
> was not wrong. That's why the problem looks a little bit complicated
> and why it would be much simpler if we stick to modern devices.
> 
> > Even if the spec is out, this is
> > pretty new and I suspect not all vendor would follow right away. I hope
> > the software framework can be tolerant with h/w vendors not supporting
> > host endianess (BE specifically) or not detecting it if they would like
> > to support a transitional device for legacy.
> 
> Well, if we know we don't want to support the BE host it would be fine.

I think you guys mean guest not host here. Same for memory ordering etc.
What matters is whether guest has barriers etc.

> >
> > > 2) bypass IOMMU with translated requests
> > > 3) PIO port
> > >
> > > Yes we have enp_vdpa, but it's more like a "transitional device" for
> > > legacy only guests.
> > >
> > >> meaning guest not acknowledging
> > >>>> VERSION_1 would use the legacy interfaces captured in the spec section 7.4
> > >>>> (regarding ring layout, native endianness, message framing, vq alignment of
> > >>>> 4096, 32bit feature, no features_ok bit in status, IO port interface i.e.
> > >>>> all the things) instead?
> > > Note that we only care about the datapath, control path is mediated anyhow.
> > >
> > > So feature_ok and IO port isn't an issue. The rest looks like a must
> > > for the hardware.
> > H/W vendors can opt out not implementing transitional interfaces at all
> > which limits itself a modern only device. Set endianess detection (via
> > transport specific means) aside, for vendors that wishes to support
> > transitional device with legacy interface, is it a hard stop to drop
> > supporting BE host if everything else is there? The spec today doesn't
> > define virtio specific means to detect host memory ordering or device
> > memory coherency,
> 
> Any reason that we need to care about memory coherency at the virtio
> level. I'd expect it's the task of transport.
> 
> > will it yet become a stopper another day for h/w
> > vendor to support more platforms?
> 
> Let's differentiate virtio from vdpa here. For virtio, there's no way
> to add any feature for legacy devices. We can only add memory features
> detecting for modern devices.
> 
> But for vDPA, we can introduce any API that can help vendors to
> present a transitional device. But we can force those APIs since it's
> too late to do that. So transitional devices support is optional for
> sure.
> 
> >
> > >
> > >> Noted we don't yet have a set_device_features()
> > >>>> that allows the vdpa device to tell whether it is operating in transitional
> > >>>> or modern-only mode.
> > > So the device feature should be provisioned via the netlink protocol.
> > Such netlink interface will only be used to limit feature exposure,
> > right? i.e. you can limit a transitional supporting vendor driver to
> > offering modern-only interface,
> 
> There's no way for the management to force a feature, like VERSION_1
> via the current protocol.
> 
> > but you never want to make a modern-only
> > vendor driver to support transitional (I'm not sure if it's a good idea
> > to support all the translation in software, esp. for datapath).
> 
> You may hit this problem for sure, you can't force all vendors to
> support transitional devices especially considering spec said legacy
> is optional. We don't want to end up with a userspace code that can
> only work for some specific vendors.
> 
> > > And what we want is not "set_device_feature()" but
> > > "set_device_mandatory_feautre()", then the parent can choose to fail
> > > the negotiation when VERSION_1 is not negotiated.
> > This assumes the transport specific detection of BE host is in place,
> > right?
> 
> Again, the point is, we can not assume such detection works for all of
> the vendors. And assume BE detection is ready, we still need this for
> modern devices, isn't it?
> 
> > I am not clear who initiates the set_device_mandatory_feautre()
> > call, QEMU during guest feature negotiation, or admin user setting it
> > ahead via netlink?
> 
> Netlink, actually, the spec needs to be extended as well, we saw
> similar requests in the past. E.g there could be a device that works
> in a packed layout only.
> 
> Thanks
> 
> >
> > Thanks,
> > -Siwei
> >
> > >   Qemu then knows for
> > > sure it talks to a transitional device or modern only device.
> > >
> > > Thanks
> > >
> > >> For software virtio, all support for the legacy part in
> > >>>> a transitional model has been built up there already, however, it's not easy
> > >>>> for vDPA vendors to implement all the requirements for an all-or-nothing
> > >>>> legacy guest support (big endian guest for example). To these vendors, the
> > >>>> legacy support within a transitional model is more of feature to them and
> > >>>> it's best to leave some flexibility for them to implement partial support
> > >>>> for legacy. That in turn calls out the need for a vhost-user protocol
> > >>>> feature like negotiation API that can prohibit those unsupported guest
> > >>>> setups to as early as backend_init before launching the VM.
> > >>> Right. Of note is the fact that it's a spec bug which I
> > >>> hope yet to fix, though due to existing guest code the
> > >>> fix won't be complete.
> > >> I thought at one point you pointed out to me that the spec does allow
> > >> config space read before claiming features_ok, and only config write
> > >> before features_ok is prohibited. I haven't read up the full thread of
> > >> Halil's VERSION_1 for transitional big endian device yet, but what is
> > >> the spec bug you hope to fix?
> > >>
> > >>> WRT ioctls, One thing we can do though is abuse set_features
> > >>> where it's called by QEMU early on with just the VERSION_1
> > >>> bit set, to distinguish between legacy and modern
> > >>> interface. This before config space accesses and FEATURES_OK.
> > >>>
> > >>> Halil has been working on this, pls take a look and maybe help him out.
> > >> Interesting thread, am reading now and see how I may leverage or help there.
> > >>
> > >>>>>>>> I
> > >>>>>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> > >>>>>>>> 0.95 support, it seems all the ingredient had been there already dated back
> > >>>>>>>> to the DPDK days. The only major thing limiting is in the vDPA software that
> > >>>>>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> > >>>>>>>> a few DMA setup ops, which is virtio 1.0 only.
> > >>>>>>>>
> > >>>>>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
> > >>>>>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
> > >>>>>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
> > >>>>>>>>>> of vdpa_get_config() to vdpa_set_config().
> > >>>>>>>>>>
> > >>>>>>>>>>              /*
> > >>>>>>>>>>               * Config accesses aren't supposed to trigger before features are
> > >>>>>>>>>> set.
> > >>>>>>>>>>               * If it does happen we assume a legacy guest.
> > >>>>>>>>>>               */
> > >>>>>>>>>>              if (!vdev->features_valid)
> > >>>>>>>>>>                      vdpa_set_features(vdev, 0);
> > >>>>>>>>>>              ops->get_config(vdev, offset, buf, len);
> > >>>>>>>>>>
> > >>>>>>>>>> I can post a patch to fix 2) if there's consensus already reached.
> > >>>>>>>>>>
> > >>>>>>>>>> Thanks,
> > >>>>>>>>>> -Siwei
> > >>>>>>>>> I'm not sure how important it is to change that.
> > >>>>>>>>> In any case it only affects transitional devices, right?
> > >>>>>>>>> Legacy only should not care ...
> > >>>>>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> > >>>>>>>> modern one in a transitional device model rather than being legacy only.
> > >>>>>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> > >>>>>>>> guests without having to reconfigure. Or are you suggesting limit to legacy
> > >>>>>>>> only at the time of vdpa creation would simplify the implementation a lot?
> > >>>>>>>>
> > >>>>>>>> Thanks,
> > >>>>>>>> -Siwei
> > >>>>>>> I don't know for sure. Take a look at the work Halil was doing
> > >>>>>>> to try and support transitional devices with BE guests.
> > >>>>>> Hmmm, we can have those endianness ioctls defined but the initial QEMU
> > >>>>>> implementation can be started to support x86 guest/host with little
> > >>>>>> endian and weak memory ordering first. The real trick is to detect
> > >>>>>> legacy guest - I am not sure if it's feasible to shift all the legacy
> > >>>>>> detection work to QEMU, or the kernel has to be part of the detection
> > >>>>>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> > >>>>>> effort in QEMU) as well. Let me take a further look and get back.
> > >>>>> Michael may think differently but I think doing this in Qemu is much easier.
> > >>>> I think the key is whether we position emulating legacy interfaces in QEMU
> > >>>> doing translation on top of a v1.0 modern-only device in the kernel, or we
> > >>>> allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a
> > >>>> transitional model in the kernel that is able to work for both v0.95 and
> > >>>> v1.0 drivers, with some slight aid from QEMU for
> > >>>> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the
> > >>>> former we still rely on vendor for a performant data vqs implementation,
> > >>>> leaving the question to what it may end up eventually in the kernel is
> > >>>> effectively the latter).
> > >>>>
> > >>>> Thanks,
> > >>>> -Siwei
> > >>> My suggestion is post the kernel patches, and we can evaluate
> > >>> how much work they are.
> > >> Thanks for the feedback. I will take some read then get back, probably
> > >> after the winter break. Stay tuned.
> > >>
> > >> Thanks,
> > >> -Siwei
> > >>
> > >>>>> Thanks
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>>> Meanwhile, I'll check internally to see if a legacy only model would
> > >>>>>> work. Thanks.
> > >>>>>>
> > >>>>>> Thanks,
> > >>>>>> -Siwei
> > >>>>>>
> > >>>>>>
> > >>>>>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
> > >>>>>>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> > >>>>>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> > >>>>>>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> > >>>>>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> > >>>>>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
> > >>>>>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
> > >>>>>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
> > >>>>>>>>>>>>>>> know what the use
> > >>>>>>>>>>>>>>> case there will be for kernel to leverage such info
> > >>>>>>>>>>>>>>> directly? Is there a
> > >>>>>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
> > >>>>>>>>>>>>>>> differentiation (legacy v.s. modern) needed?
> > >>>>>>>>>>>>>> BTW a good API could be
> > >>>>>>>>>>>>>>
> > >>>>>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > >>>>>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> > >>>>>>>>>>>>>>
> > >>>>>>>>>>>>>> we did it per vring but maybe that was a mistake ...
> > >>>>>>>>>>>>> Actually, I wonder whether it's good time to just not support
> > >>>>>>>>>>>>> legacy driver
> > >>>>>>>>>>>>> for vDPA. Consider:
> > >>>>>>>>>>>>>
> > >>>>>>>>>>>>> 1) It's definition is no-normative
> > >>>>>>>>>>>>> 2) A lot of budren of codes
> > >>>>>>>>>>>>>
> > >>>>>>>>>>>>> So qemu can still present the legacy device since the config
> > >>>>>>>>>>>>> space or other
> > >>>>>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
> > >>>>>>>>>>>>> accessed by
> > >>>>>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
> > >>>>>>>>>>>>> in this
> > >>>>>>>>>>>>> case?
> > >>>>>>>>>>>>>
> > >>>>>>>>>>>>> Thanks
> > >>>>>>>>>>>>>
> > >>>>>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
> > >>>>>>>>>>>> working userspace, qemu releases with vdpa support are out there and
> > >>>>>>>>>>>> seem to work for people. Any changes need to take that into account
> > >>>>>>>>>>>> and document compatibility concerns.
> > >>>>>>>>>>> Agree, let me check.
> > >>>>>>>>>>>
> > >>>>>>>>>>>
> > >>>>>>>>>>>>        I note that any hardware
> > >>>>>>>>>>>> implementation is already broken for legacy except on platforms with
> > >>>>>>>>>>>> strong ordering which might be helpful in reducing the scope.
> > >>>>>>>>>>> Yes.
> > >>>>>>>>>>>
> > >>>>>>>>>>> Thanks
> > >>>>>>>>>>>
> > >>>>>>>>>>>
> >

Jason Wang Dec. 17, 2021, 2:01 a.m. UTC | #86

On Fri, Dec 17, 2021 at 9:08 AM Si-Wei Liu <[e-mail]> wrote:
>
>
>
> On 12/15/2021 7:43 PM, Jason Wang wrote:
> > On Thu, Dec 16, 2021 at 4:52 AM Si-Wei Liu <[e-mail]> wrote:
> >>
> >>
> >> On 12/14/2021 6:06 PM, Jason Wang wrote:
> >>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
> >>>>
> >>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> >>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> >>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> >>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> >>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> >>>>>>>> it ended up with. I have the following questions,
> >>>>>>>>
> >>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
> >>>>>>>> support will be completely dropped from the table, is my understanding
> >>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> >>>>>>>> which is backed by the spec at
> >>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> >>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
> >>>>>>>> beyond.
> >>>>>>> I personally feel it's less work to add in kernel than try to
> >>>>>>> work around it in userspace. Jason feels differently.
> >>>>>>> Maybe post the patches and this will prove to Jason it's not
> >>>>>>> too terrible?
> >>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> >>>>>> layout level and is limited to x86 only, there should be easy way out.
> >>>>> Note a subtle difference: what matters is that guest, not host is x86.
> >>>>> Matters for emulators which might reorder memory accesses.
> >>>>> I guess this enforcement belongs in QEMU then?
> >>>> Right, I mean to get started, the initial guest driver support and the
> >>>> corresponding QEMU support for transitional vdpa backend can be limited
> >>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
> >>>> suppose it's not hard to enforce in QEMU.
> >>> It's more than just config space, most devices have headers before the buffer.
> >> The ordering in datapath (data VQs) would have to rely on vendor's
> >> support. Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w
> >> vendor nowadays can/should well support the case when ORDER_PLATFORM is
> >> not acked by the driver (actually this feature is filtered out by the
> >> QEMU vhost-vdpa driver today), even with v1.0 spec conforming and modern
> >> only vDPA device.
> > That's a bug that needs to be fixed.
> >
> >> The control VQ is implemented in software in the
> >> kernel, which can be easily accommodated/fixed when needed.
> >>
> >>>> QEMU can drive GET_LEGACY,
> >>>> GET_ENDIAN et al ioctls in advance to get the capability from the
> >>>> individual vendor driver. For that, we need another negotiation protocol
> >>>> similar to vhost_user's protocol_features between the vdpa kernel and
> >>>> QEMU, way before the guest driver is ever probed and its feature
> >>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> >>>> from the device, but we can assume weak ordering for legacy at this
> >>>> point (x86 only)?
> >>> I'm lost here, we have get_features() so:
> >> I assume here you refer to get_device_features() that Eli just changed
> >> the name.
> >>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> >>> 2) ORDER_PLATFORM means device requires platform ordering
> >>>
> >>> Any reason for having a new API for this?
> >> Are you going to enforce all vDPA hardware vendors to support the
> >> transitional model for legacy guest? meaning guest not acknowledging
> >> VERSION_1 would use the legacy interfaces captured in the spec section
> >> 7.4 (regarding ring layout, native endianness, message framing, vq
> >> alignment of 4096, 32bit feature, no features_ok bit in status, IO port
> >> interface i.e. all the things) instead? Noted we don't yet have a
> >> set_device_features() that allows the vdpa device to tell whether it is
> >> operating in transitional or modern-only mode. For software virtio, all
> >> support for the legacy part in a transitional model has been built up
> >> there already, however, it's not easy for vDPA vendors to implement all
> >> the requirements for an all-or-nothing legacy guest support (big endian
> >> guest for example). To these vendors, the legacy support within a
> >> transitional model is more of feature to them and it's best to leave
> >> some flexibility for them to implement partial support for legacy. That
> >> in turn calls out the need for a vhost-user protocol feature like
> >> negotiation API that can prohibit those unsupported guest setups to as
> >> early as backend_init before launching the VM.
> >>
> >>
> >>>>>> I
> >>>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> >>>>>> 0.95 support, it seems all the ingredient had been there already dated back
> >>>>>> to the DPDK days. The only major thing limiting is in the vDPA software that
> >>>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> >>>>>> a few DMA setup ops, which is virtio 1.0 only.
> >>>>>>
> >>>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we
> >>>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
> >>>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
> >>>>>>>> of vdpa_get_config() to vdpa_set_config().
> >>>>>>>>
> >>>>>>>>             /*
> >>>>>>>>              * Config accesses aren't supposed to trigger before features are
> >>>>>>>> set.
> >>>>>>>>              * If it does happen we assume a legacy guest.
> >>>>>>>>              */
> >>>>>>>>             if (!vdev->features_valid)
> >>>>>>>>                     vdpa_set_features(vdev, 0);
> >>>>>>>>             ops->get_config(vdev, offset, buf, len);
> >>>>>>>>
> >>>>>>>> I can post a patch to fix 2) if there's consensus already reached.
> >>>>>>>>
> >>>>>>>> Thanks,
> >>>>>>>> -Siwei
> >>>>>>> I'm not sure how important it is to change that.
> >>>>>>> In any case it only affects transitional devices, right?
> >>>>>>> Legacy only should not care ...
> >>>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> >>>>>> modern one in a transitional device model rather than being legacy only.
> >>>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> >>>>>> guests without having to reconfigure. Or are you suggesting limit to legacy
> >>>>>> only at the time of vdpa creation would simplify the implementation a lot?
> >>>>>>
> >>>>>> Thanks,
> >>>>>> -Siwei
> >>>>> I don't know for sure. Take a look at the work Halil was doing
> >>>>> to try and support transitional devices with BE guests.
> >>>> Hmmm, we can have those endianness ioctls defined but the initial QEMU
> >>>> implementation can be started to support x86 guest/host with little
> >>>> endian and weak memory ordering first. The real trick is to detect
> >>>> legacy guest - I am not sure if it's feasible to shift all the legacy
> >>>> detection work to QEMU, or the kernel has to be part of the detection
> >>>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> >>>> effort in QEMU) as well. Let me take a further look and get back.
> >>> Michael may think differently but I think doing this in Qemu is much easier.
> >> I think the key is whether we position emulating legacy interfaces in
> >> QEMU doing translation on top of a v1.0 modern-only device in the
> >> kernel, or we allow vdpa core (or you can say vhost-vdpa) and vendor
> >> driver to support a transitional model in the kernel that is able to
> >> work for both v0.95 and v1.0 drivers, with some slight aid from QEMU for
> >> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for
> >> the former we still rely on vendor for a performant data vqs
> >> implementation, leaving the question to what it may end up eventually in
> >> the kernel is effectively the latter).
> > I think we can do the legacy interface emulation on top of the shadow
> > VQ. And we know it works for sure. But I agree, it would be much
> > easier if we depend on the vendor to implement a transitional device.
> First I am not sure if there's a convincing case for users to deploy
> vDPA with shadow (data) VQ against the pure software based backend.
> Please enlighten me if there is.

The problem is shadow VQ is the only solution that can works for all the cases.

>
> For us, the point to deploy vDPA for legacy guest is the acceleration
> (what "A" stands for in "vDPA") part of it so that we can leverage the
> hardware potential if at all possible. Not sure how the shadow VQ
> implementation can easily deal with datapath acceleration without losing
> too much performance?

It's not easy, shadow VQ will lose performance for sure.

>
> > So assuming we depend on the vendor, I don't see anything that is
> > strictly needed in the kernel, the kick or config access before
> > DRIVER_OK can all be handled easily in Qemu unless I miss something.
> Right, that's what I think too it's not quite a lot of work in the
> kernel if vendor device offers the aid/support for transitional. The
> kernel only provides the abstraction of device model (transitional or
> modern-only), while vendor driver may implement early platform feature
> discovery and apply legacy specific quirks (unsupported endianness,
> mismatched page size, unsupported host memory ordering model) that the
> device can't adapt to. I don't say we have to depend on the vendor, but
> the point is that we must assume fully spec compliant transitional
> support (the datapath in particular) from the vendor to get started, as
> I guess it's probably the main motivation for users to deploy it -
> acceleration of legacy guest workload without exhausting host computing
> resource. Even if we get started with shadow VQ to mediate and translate
> the datapath, eventually it may evolve towards leveraging datapath
> offload to hardware if acceleration is the only convincing use case for
> legacy support.

Yes, so as discussed, I don't object the idea, kernel patches are more
than welcomed.

Thanks

>
> Thanks,
> -Siwei
> > The only value to do that in the kernel is that it can work for
> > virtio-vdpa, but modern only virito-vpda is sufficient; we don't need
> > any legacy stuff for that.
> >
> > Thanks
> >
> >> Thanks,
> >> -Siwei
> >>
> >>> Thanks
> >>>
> >>>
> >>>
> >>>> Meanwhile, I'll check internally to see if a legacy only model would
> >>>> work. Thanks.
> >>>>
> >>>> Thanks,
> >>>> -Siwei
> >>>>
> >>>>
> >>>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
> >>>>>>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> >>>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> >>>>>>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> >>>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> >>>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
> >>>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
> >>>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I
> >>>>>>>>>>>>> know what the use
> >>>>>>>>>>>>> case there will be for kernel to leverage such info
> >>>>>>>>>>>>> directly? Is there a
> >>>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
> >>>>>>>>>>>>> differentiation (legacy v.s. modern) needed?
> >>>>>>>>>>>> BTW a good API could be
> >>>>>>>>>>>>
> >>>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>>>>>
> >>>>>>>>>>>> we did it per vring but maybe that was a mistake ...
> >>>>>>>>>>> Actually, I wonder whether it's good time to just not support
> >>>>>>>>>>> legacy driver
> >>>>>>>>>>> for vDPA. Consider:
> >>>>>>>>>>>
> >>>>>>>>>>> 1) It's definition is no-normative
> >>>>>>>>>>> 2) A lot of budren of codes
> >>>>>>>>>>>
> >>>>>>>>>>> So qemu can still present the legacy device since the config
> >>>>>>>>>>> space or other
> >>>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
> >>>>>>>>>>> accessed by
> >>>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary
> >>>>>>>>>>> in this
> >>>>>>>>>>> case?
> >>>>>>>>>>>
> >>>>>>>>>>> Thanks
> >>>>>>>>>>>
> >>>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking
> >>>>>>>>>> working userspace, qemu releases with vdpa support are out there and
> >>>>>>>>>> seem to work for people. Any changes need to take that into account
> >>>>>>>>>> and document compatibility concerns.
> >>>>>>>>> Agree, let me check.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>>       I note that any hardware
> >>>>>>>>>> implementation is already broken for legacy except on platforms with
> >>>>>>>>>> strong ordering which might be helpful in reducing the scope.
> >>>>>>>>> Yes.
> >>>>>>>>>
> >>>>>>>>> Thanks
> >>>>>>>>>
> >>>>>>>>>
>

Jason Wang Dec. 17, 2021, 2:15 a.m. UTC | #87

On Fri, Dec 17, 2021 at 10:01 AM Michael S. Tsirkin <[e-mail]> wrote:
>
> On Fri, Dec 17, 2021 at 09:57:38AM +0800, Jason Wang wrote:
> > On Fri, Dec 17, 2021 at 6:32 AM Si-Wei Liu <[e-mail]> wrote:
> > >
> > >
> > >
> > > On 12/15/2021 6:53 PM, Jason Wang wrote:
> > > > On Thu, Dec 16, 2021 at 10:02 AM Si-Wei Liu <[e-mail]> wrote:
> > > >>
> > > >>
> > > >> On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:
> > > >>> On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
> > > >>>> On 12/14/2021 6:06 PM, Jason Wang wrote:
> > > >>>>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <[e-mail]> wrote:
> > > >>>>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> > > >>>>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> > > >>>>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> > > >>>>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > > >>>>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > > >>>>>>>>>> it ended up with. I have the following questions,
> > > >>>>>>>>>>
> > > >>>>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
> > > >>>>>>>>>> support will be completely dropped from the table, is my understanding
> > > >>>>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > > >>>>>>>>>> which is backed by the spec at
> > > >>>>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> > > >>>>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
> > > >>>>>>>>>> beyond.
> > > >>>>>>>>> I personally feel it's less work to add in kernel than try to
> > > >>>>>>>>> work around it in userspace. Jason feels differently.
> > > >>>>>>>>> Maybe post the patches and this will prove to Jason it's not
> > > >>>>>>>>> too terrible?
> > > >>>>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> > > >>>>>>>> layout level and is limited to x86 only, there should be easy way out.
> > > >>>>>>> Note a subtle difference: what matters is that guest, not host is x86.
> > > >>>>>>> Matters for emulators which might reorder memory accesses.
> > > >>>>>>> I guess this enforcement belongs in QEMU then?
> > > >>>>>> Right, I mean to get started, the initial guest driver support and the
> > > >>>>>> corresponding QEMU support for transitional vdpa backend can be limited
> > > >>>>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
> > > >>>>>> suppose it's not hard to enforce in QEMU.
> > > >>>>> It's more than just config space, most devices have headers before the buffer.
> > > >>>> The ordering in datapath (data VQs) would have to rely on vendor's support.
> > > >>>> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
> > > >>>> can/should well support the case when ORDER_PLATFORM is not acked by the
> > > >>>> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
> > > >>>> today), even with v1.0 spec conforming and modern only vDPA device. The
> > > >>>> control VQ is implemented in software in the kernel, which can be easily
> > > >>>> accommodated/fixed when needed.
> > > >>>>
> > > >>>>>> QEMU can drive GET_LEGACY,
> > > >>>>>> GET_ENDIAN et al ioctls in advance to get the capability from the
> > > >>>>>> individual vendor driver. For that, we need another negotiation protocol
> > > >>>>>> similar to vhost_user's protocol_features between the vdpa kernel and
> > > >>>>>> QEMU, way before the guest driver is ever probed and its feature
> > > >>>>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> > > >>>>>> from the device, but we can assume weak ordering for legacy at this
> > > >>>>>> point (x86 only)?
> > > >>>>> I'm lost here, we have get_features() so:
> > > >>>> I assume here you refer to get_device_features() that Eli just changed the
> > > >>>> name.
> > > >>>>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> > > >>>>> 2) ORDER_PLATFORM means device requires platform ordering
> > > >>>>>
> > > >>>>> Any reason for having a new API for this?
> > > >>>> Are you going to enforce all vDPA hardware vendors to support the
> > > >>>> transitional model for legacy guest?
> > > > Do we really have other choices?
> > > >
> > > > I suspect the legacy device is never implemented by any vendor:
> > > >
> > > > 1) no virtio way to detect host endian
> > > This is even true for transitional device that is conforming to the
> > > spec, right?
> >
> > For hardware, yes.
> >
> > > The transport specific way to detect host endian is still
> > > being discussed and the spec revision is not finalized yet so far as I
> > > see. Why this suddenly becomes a requirement/blocker for h/w vendors to
> > > implement the transitional model?
> >
> > It's not a sudden blocker, the problem has existed since day 0 if I
> > was not wrong. That's why the problem looks a little bit complicated
> > and why it would be much simpler if we stick to modern devices.
> >
> > > Even if the spec is out, this is
> > > pretty new and I suspect not all vendor would follow right away. I hope
> > > the software framework can be tolerant with h/w vendors not supporting
> > > host endianess (BE specifically) or not detecting it if they would like
> > > to support a transitional device for legacy.
> >
> > Well, if we know we don't want to support the BE host it would be fine.
>
> I think you guys mean guest not host here. Same for memory ordering etc.
> What matters is whether guest has barriers etc.
>

Yes.

Thanks

Patch

diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index 7c1f789..540dd67 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
@@ -1490,14 +1490,6 @@  static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
 	return mvdev->mlx_features;
 }
 
-static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
-{
-	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
-		return -EOPNOTSUPP;
-
-	return 0;
-}
-
 static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
 {
 	int err;
@@ -1558,18 +1550,13 @@  static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
 {
 	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
 	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
-	int err;
 
 	print_features(mvdev, features, true);
 
-	err = verify_min_features(mvdev, features);
-	if (err)
-		return err;
-
 	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
 	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
 	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
-	return err;
+	return 0;
 }
 
 static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)