[v3,4/9] s390: ap: tools to find a queue with a specific APQN

Commit Message

Pierre Morel Feb. 14, 2019, 1:51 p.m. UTC

We need to find the queue with a specific APQN during the
handling of the interception of the PQAP/AQIC instruction.

To handle the AP associated device reference count we keep
track of it in the vfio_ap_queue until we put the device.

Signed-off-by: Pierre Morel <[e-mail]>
---
 drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
 drivers/s390/crypto/vfio_ap_private.h |  1 +
 2 files changed, 55 insertions(+)

Comments

Cornelia Huck Feb. 15, 2019, 9:49 a.m. UTC | #1

On Thu, 14 Feb 2019 14:51:04 +0100
Pierre Morel <[e-mail]> wrote:

> We need to find the queue with a specific APQN during the
> handling of the interception of the PQAP/AQIC instruction.
> 
> To handle the AP associated device reference count we keep
> track of it in the vfio_ap_queue until we put the device.

So, the relationship is
(struct ap_device)--(driver_data)-->(struct vfio_ap_queue)--(pointer)-->(struct ap_device)
? IOW, a backlink?

If so, can't you already set that up during probe?

Or am I confused by the various similar devices again? Maybe a diagram
would help...

> 
> Signed-off-by: Pierre Morel <[e-mail]>
> ---
>  drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
>  drivers/s390/crypto/vfio_ap_private.h |  1 +
>  2 files changed, 55 insertions(+)
> 
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index 900b9cf..2a52c9b 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -24,6 +24,60 @@
>  #define VFIO_AP_MDEV_TYPE_HWVIRT "passthrough"
>  #define VFIO_AP_MDEV_NAME_HWVIRT "VFIO AP Passthrough Device"
>  
> +/**
> + * vfio_ap_check_apqn: check if a ap_queue is of a given APQN
> + *
> + * Returns 1 if we have a match.
> + * Otherwise returns 0.
> + */
> +static int vfio_ap_check_apqn(struct device *dev, void *data)
> +{
> +	struct vfio_ap_queue *q = dev_get_drvdata(dev);
> +
> +	return (q->apqn == *(int *)data);
> +}
> +
> +/**
> + * vfio_ap_get_queue: Retrieve a queue with a specific APQN
> + * @apqn: The queue APQN
> + *
> + * Retrieve a queue with a specific APQN from the list of the
> + * devices associated to the vfio_ap_driver.
> + *
> + * The vfio_ap_queue has been already associated with the device
> + * during the probe.
> + * Store the associated device for reference counting
> + *
> + * Returns the pointer to the associated vfio_ap_queue
> + */
> +static  __attribute__((unused))

Eww. Can you get rid of that by reordering or squashing patches?

> +	struct vfio_ap_queue *vfio_ap_get_queue(int apqn)
> +{
> +	struct device *dev;
> +	struct vfio_ap_queue *q;
> +
> +	dev = driver_find_device(&matrix_dev->vfio_ap_drv->driver, NULL, &apqn,
> +				 vfio_ap_check_apqn);
> +	if (!dev)
> +		return NULL;
> +	q = dev_get_drvdata(dev);
> +	q->dev = dev;
> +	return q;
> +}
> +
> +/**
> + * vfio_ap_put_queue: lower device reference count for a queue
> + * @q: The queue
> + *
> + * put the associated device
> + *
> + */
> +static  __attribute__((unused)) void vfio_ap_put_queue(struct vfio_ap_queue *q)
> +{
> +	put_device(q->dev);
> +	q->dev = NULL;
> +}
> +
>  static void vfio_ap_matrix_init(struct ap_config_info *info,
>  				struct ap_matrix *matrix)
>  {
> diff --git a/drivers/s390/crypto/vfio_ap_private.h b/drivers/s390/crypto/vfio_ap_private.h
> index 8836f01..081f0d7 100644
> --- a/drivers/s390/crypto/vfio_ap_private.h
> +++ b/drivers/s390/crypto/vfio_ap_private.h
> @@ -87,6 +87,7 @@ extern int vfio_ap_mdev_register(void);
>  extern void vfio_ap_mdev_unregister(void);
>  
>  struct vfio_ap_queue {
> +	struct device *dev;
>  	int	apqn;
>  };
>  #endif /* _VFIO_AP_PRIVATE_H_ */

Pierre Morel Feb. 15, 2019, 10:10 a.m. UTC | #2

On 15/02/2019 10:49, Cornelia Huck wrote:
> On Thu, 14 Feb 2019 14:51:04 +0100
> Pierre Morel <[e-mail]> wrote:
> 
>> We need to find the queue with a specific APQN during the
>> handling of the interception of the PQAP/AQIC instruction.
>>
>> To handle the AP associated device reference count we keep
>> track of it in the vfio_ap_queue until we put the device.
> 
> So, the relationship is
> (struct ap_device)--(driver_data)-->(struct vfio_ap_queue)--(pointer)-->(struct ap_device)
> ? IOW, a backlink?
> 
> If so, can't you already set that up during probe?

Will do.

> 
> Or am I confused by the various similar devices again? Maybe a diagram
> would help...

No you are right.


> 
>>
>> Signed-off-by: Pierre Morel <[e-mail]>
>> ---
>>   drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
>>   drivers/s390/crypto/vfio_ap_private.h |  1 +
>>   2 files changed, 55 insertions(+)
>>
>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
>> index 900b9cf..2a52c9b 100644
>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>> @@ -24,6 +24,60 @@
>>   #define VFIO_AP_MDEV_TYPE_HWVIRT "passthrough"
>>   #define VFIO_AP_MDEV_NAME_HWVIRT "VFIO AP Passthrough Device"
>>   
>> +/**
>> + * vfio_ap_check_apqn: check if a ap_queue is of a given APQN
>> + *
>> + * Returns 1 if we have a match.
>> + * Otherwise returns 0.
>> + */
>> +static int vfio_ap_check_apqn(struct device *dev, void *data)
>> +{
>> +	struct vfio_ap_queue *q = dev_get_drvdata(dev);
>> +
>> +	return (q->apqn == *(int *)data);
>> +}
>> +
>> +/**
>> + * vfio_ap_get_queue: Retrieve a queue with a specific APQN
>> + * @apqn: The queue APQN
>> + *
>> + * Retrieve a queue with a specific APQN from the list of the
>> + * devices associated to the vfio_ap_driver.
>> + *
>> + * The vfio_ap_queue has been already associated with the device
>> + * during the probe.
>> + * Store the associated device for reference counting
>> + *
>> + * Returns the pointer to the associated vfio_ap_queue
>> + */
>> +static  __attribute__((unused))
> 
> Eww. Can you get rid of that by reordering or squashing patches?

I did this to avoid posting a very big patch.
I will of course squash 4 and 5 with patch 6, when the two patches 4 and 
5 are reviewed.

If you think it brings more clarity to squash all for the next iteration 
I will do.

Regards,
Pierre

Cornelia Huck Feb. 15, 2019, 10:24 a.m. UTC | #3

On Fri, 15 Feb 2019 11:10:43 +0100
Pierre Morel <[e-mail]> wrote:

> On 15/02/2019 10:49, Cornelia Huck wrote:
> > On Thu, 14 Feb 2019 14:51:04 +0100
> > Pierre Morel <[e-mail]> wrote:
> >   
> >> We need to find the queue with a specific APQN during the
> >> handling of the interception of the PQAP/AQIC instruction.
> >>
> >> To handle the AP associated device reference count we keep
> >> track of it in the vfio_ap_queue until we put the device.  
> > 
> > So, the relationship is
> > (struct ap_device)--(driver_data)-->(struct vfio_ap_queue)--(pointer)-->(struct ap_device)
> > ? IOW, a backlink?
> > 
> > If so, can't you already set that up during probe?  
> 
> Will do.
> 
> > 
> > Or am I confused by the various similar devices again? Maybe a diagram
> > would help...  
> 
> No you are right.

Good, I was fearing that I was more confused than normal for Fridays ;)

> 
> 
> >   
> >>
> >> Signed-off-by: Pierre Morel <[e-mail]>
> >> ---
> >>   drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
> >>   drivers/s390/crypto/vfio_ap_private.h |  1 +
> >>   2 files changed, 55 insertions(+)
> >>
> >> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> >> index 900b9cf..2a52c9b 100644
> >> --- a/drivers/s390/crypto/vfio_ap_ops.c
> >> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> >> @@ -24,6 +24,60 @@
> >>   #define VFIO_AP_MDEV_TYPE_HWVIRT "passthrough"
> >>   #define VFIO_AP_MDEV_NAME_HWVIRT "VFIO AP Passthrough Device"
> >>   
> >> +/**
> >> + * vfio_ap_check_apqn: check if a ap_queue is of a given APQN
> >> + *
> >> + * Returns 1 if we have a match.
> >> + * Otherwise returns 0.
> >> + */
> >> +static int vfio_ap_check_apqn(struct device *dev, void *data)
> >> +{
> >> +	struct vfio_ap_queue *q = dev_get_drvdata(dev);
> >> +
> >> +	return (q->apqn == *(int *)data);
> >> +}
> >> +
> >> +/**
> >> + * vfio_ap_get_queue: Retrieve a queue with a specific APQN
> >> + * @apqn: The queue APQN
> >> + *
> >> + * Retrieve a queue with a specific APQN from the list of the
> >> + * devices associated to the vfio_ap_driver.
> >> + *
> >> + * The vfio_ap_queue has been already associated with the device
> >> + * during the probe.
> >> + * Store the associated device for reference counting
> >> + *
> >> + * Returns the pointer to the associated vfio_ap_queue
> >> + */
> >> +static  __attribute__((unused))  
> > 
> > Eww. Can you get rid of that by reordering or squashing patches?  
> 
> I did this to avoid posting a very big patch.
> I will of course squash 4 and 5 with patch 6, when the two patches 4 and 
> 5 are reviewed.
> 
> If you think it brings more clarity to squash all for the next iteration 
> I will do.

Let's just see what the patches look like in the end. If a squashed
patch is not too unwieldy, I'd prefer that over those unused
annotations, though.

Hoping for review from others as well ;)

Anthony Krowiak Feb. 15, 2019, 10:13 p.m. UTC | #4

On 2/14/19 8:51 AM, Pierre Morel wrote:
> We need to find the queue with a specific APQN during the
> handling of the interception of the PQAP/AQIC instruction.
> 
> To handle the AP associated device reference count we keep
> track of it in the vfio_ap_queue until we put the device.
> 
> Signed-off-by: Pierre Morel <[e-mail]>
> ---
>   drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
>   drivers/s390/crypto/vfio_ap_private.h |  1 +
>   2 files changed, 55 insertions(+)
> 
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index 900b9cf..2a52c9b 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -24,6 +24,60 @@
>   #define VFIO_AP_MDEV_TYPE_HWVIRT "passthrough"
>   #define VFIO_AP_MDEV_NAME_HWVIRT "VFIO AP Passthrough Device"
>   
> +/**
> + * vfio_ap_check_apqn: check if a ap_queue is of a given APQN
> + *
> + * Returns 1 if we have a match.
> + * Otherwise returns 0.
> + */
> +static int vfio_ap_check_apqn(struct device *dev, void *data)
> +{
> +	struct vfio_ap_queue *q = dev_get_drvdata(dev);
> +
> +	return (q->apqn == *(int *)data);
> +}
> +
> +/**
> + * vfio_ap_get_queue: Retrieve a queue with a specific APQN
> + * @apqn: The queue APQN
> + *
> + * Retrieve a queue with a specific APQN from the list of the
> + * devices associated to the vfio_ap_driver.
> + *
> + * The vfio_ap_queue has been already associated with the device
> + * during the probe.
> + * Store the associated device for reference counting
> + *
> + * Returns the pointer to the associated vfio_ap_queue
> + */
> +static  __attribute__((unused))
> +	struct vfio_ap_queue *vfio_ap_get_queue(int apqn)

I think you should change this signature for the reasons I stated
below:

struct device *vfio_ap_get_queue_dev(int apqn)

> +{
> +	struct device *dev;
> +	struct vfio_ap_queue *q;
> +
> +	dev = driver_find_device(&matrix_dev->vfio_ap_drv->driver, NULL, &apqn,
> +				 vfio_ap_check_apqn);
> +	if (!dev)
> +		return NULL;
> +	q = dev_get_drvdata(dev);
> +	q->dev = dev;

Why store the device with the vfio_ap_queue object? Why not just return
the device. The caller can get the vfio_ap_queue from the device's
driver data. It seems the only reason for the 'dev' field is to
temporarily hold a ref to the device so it can be put later. Why not
just put the device.

> +	return q;
> +}
> +
> +/**
> + * vfio_ap_put_queue: lower device reference count for a queue
> + * @q: The queue
> + *
> + * put the associated device
> + *
> + */
> +static  __attribute__((unused)) void vfio_ap_put_queue(struct vfio_ap_queue *q)
> +{
> +	put_device(q->dev);
> +	q->dev = NULL;
> +}

I would get rid of this function. If you take my suggestion above, you
can just call the put_device() directly. There will be no need for
this superfluous 'dev' field.

> +
>   static void vfio_ap_matrix_init(struct ap_config_info *info,
>   				struct ap_matrix *matrix)
>   {
> diff --git a/drivers/s390/crypto/vfio_ap_private.h b/drivers/s390/crypto/vfio_ap_private.h
> index 8836f01..081f0d7 100644
> --- a/drivers/s390/crypto/vfio_ap_private.h
> +++ b/drivers/s390/crypto/vfio_ap_private.h
> @@ -87,6 +87,7 @@ extern int vfio_ap_mdev_register(void);
>   extern void vfio_ap_mdev_unregister(void);
>   
>   struct vfio_ap_queue {
> +	struct device *dev;

No need for this as per my comments above.

>   	int	apqn;
>   };
>   #endif /* _VFIO_AP_PRIVATE_H_ */
>

Cornelia Huck Feb. 18, 2019, 12:21 p.m. UTC | #5

On Fri, 15 Feb 2019 17:13:21 -0500
Tony Krowiak <[e-mail]> wrote:

> On 2/14/19 8:51 AM, Pierre Morel wrote:
> > We need to find the queue with a specific APQN during the
> > handling of the interception of the PQAP/AQIC instruction.
> > 
> > To handle the AP associated device reference count we keep
> > track of it in the vfio_ap_queue until we put the device.
> > 
> > Signed-off-by: Pierre Morel <[e-mail]>
> > ---
> >   drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
> >   drivers/s390/crypto/vfio_ap_private.h |  1 +
> >   2 files changed, 55 insertions(+)

> > +/**
> > + * vfio_ap_get_queue: Retrieve a queue with a specific APQN
> > + * @apqn: The queue APQN
> > + *
> > + * Retrieve a queue with a specific APQN from the list of the
> > + * devices associated to the vfio_ap_driver.
> > + *
> > + * The vfio_ap_queue has been already associated with the device
> > + * during the probe.
> > + * Store the associated device for reference counting
> > + *
> > + * Returns the pointer to the associated vfio_ap_queue
> > + */
> > +static  __attribute__((unused))
> > +	struct vfio_ap_queue *vfio_ap_get_queue(int apqn)  
> 
> I think you should change this signature for the reasons I stated
> below:
> 
> struct device *vfio_ap_get_queue_dev(int apqn)
> 
> > +{
> > +	struct device *dev;
> > +	struct vfio_ap_queue *q;
> > +
> > +	dev = driver_find_device(&matrix_dev->vfio_ap_drv->driver, NULL, &apqn,
> > +				 vfio_ap_check_apqn);
> > +	if (!dev)
> > +		return NULL;
> > +	q = dev_get_drvdata(dev);
> > +	q->dev = dev;  
> 
> Why store the device with the vfio_ap_queue object? Why not just return
> the device. The caller can get the vfio_ap_queue from the device's
> driver data. It seems the only reason for the 'dev' field is to
> temporarily hold a ref to the device so it can be put later. Why not
> just put the device.

Having looked at the remainder of the patches, I tend to agree that we
don't really need the backlink; we walk the driver's list of devices in
any case IIUC.

We *might* want a mechanism to grab the queue quickly (i.e. without
walking the list) if there's anything performance sensitive in there;
but from the patch descriptions, I don't think anything is done in a
hot path, so it should be fine.

Pierre Morel Feb. 18, 2019, 6:32 p.m. UTC | #6

On 18/02/2019 13:21, Cornelia Huck wrote:
> On Fri, 15 Feb 2019 17:13:21 -0500
> Tony Krowiak <[e-mail]> wrote:
> 
>> On 2/14/19 8:51 AM, Pierre Morel wrote:
>>> We need to find the queue with a specific APQN during the
>>> handling of the interception of the PQAP/AQIC instruction.
>>>
>>> To handle the AP associated device reference count we keep
>>> track of it in the vfio_ap_queue until we put the device.
>>>
>>> Signed-off-by: Pierre Morel <[e-mail]>
>>> ---
>>>    drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
>>>    drivers/s390/crypto/vfio_ap_private.h |  1 +
>>>    2 files changed, 55 insertions(+)
> 
>>> +/**
>>> + * vfio_ap_get_queue: Retrieve a queue with a specific APQN
>>> + * @apqn: The queue APQN
>>> + *
>>> + * Retrieve a queue with a specific APQN from the list of the
>>> + * devices associated to the vfio_ap_driver.
>>> + *
>>> + * The vfio_ap_queue has been already associated with the device
>>> + * during the probe.
>>> + * Store the associated device for reference counting
>>> + *
>>> + * Returns the pointer to the associated vfio_ap_queue
>>> + */
>>> +static  __attribute__((unused))
>>> +	struct vfio_ap_queue *vfio_ap_get_queue(int apqn)
>>
>> I think you should change this signature for the reasons I stated
>> below:
>>
>> struct device *vfio_ap_get_queue_dev(int apqn)
>>
>>> +{
>>> +	struct device *dev;
>>> +	struct vfio_ap_queue *q;
>>> +
>>> +	dev = driver_find_device(&matrix_dev->vfio_ap_drv->driver, NULL, &apqn,
>>> +				 vfio_ap_check_apqn);
>>> +	if (!dev)
>>> +		return NULL;
>>> +	q = dev_get_drvdata(dev);
>>> +	q->dev = dev;
>>
>> Why store the device with the vfio_ap_queue object? Why not just return
>> the device. The caller can get the vfio_ap_queue from the device's
>> driver data. It seems the only reason for the 'dev' field is to
>> temporarily hold a ref to the device so it can be put later. Why not
>> just put the device.
> 
> Having looked at the remainder of the patches, I tend to agree that we
> don't really need the backlink; we walk the driver's list of devices in
> any case IIUC.
> 
> We *might* want a mechanism to grab the queue quickly (i.e. without
> walking the list) if there's anything performance sensitive in there;
> but from the patch descriptions, I don't think anything is done in a
> hot path, so it should be fine.
> 

OK you are right, I ll drop it

Anthony Krowiak Feb. 22, 2019, 3:04 p.m. UTC | #7

On 2/18/19 7:21 AM, Cornelia Huck wrote:
> On Fri, 15 Feb 2019 17:13:21 -0500
> Tony Krowiak <[e-mail]> wrote:
> 
>> On 2/14/19 8:51 AM, Pierre Morel wrote:
>>> We need to find the queue with a specific APQN during the
>>> handling of the interception of the PQAP/AQIC instruction.
>>>
>>> To handle the AP associated device reference count we keep
>>> track of it in the vfio_ap_queue until we put the device.
>>>
>>> Signed-off-by: Pierre Morel <[e-mail]>
>>> ---
>>>    drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
>>>    drivers/s390/crypto/vfio_ap_private.h |  1 +
>>>    2 files changed, 55 insertions(+)
> 
>>> +/**
>>> + * vfio_ap_get_queue: Retrieve a queue with a specific APQN
>>> + * @apqn: The queue APQN
>>> + *
>>> + * Retrieve a queue with a specific APQN from the list of the
>>> + * devices associated to the vfio_ap_driver.
>>> + *
>>> + * The vfio_ap_queue has been already associated with the device
>>> + * during the probe.
>>> + * Store the associated device for reference counting
>>> + *
>>> + * Returns the pointer to the associated vfio_ap_queue
>>> + */
>>> +static  __attribute__((unused))
>>> +	struct vfio_ap_queue *vfio_ap_get_queue(int apqn)
>>
>> I think you should change this signature for the reasons I stated
>> below:
>>
>> struct device *vfio_ap_get_queue_dev(int apqn)
>>
>>> +{
>>> +	struct device *dev;
>>> +	struct vfio_ap_queue *q;
>>> +
>>> +	dev = driver_find_device(&matrix_dev->vfio_ap_drv->driver, NULL, &apqn,
>>> +				 vfio_ap_check_apqn);
>>> +	if (!dev)
>>> +		return NULL;
>>> +	q = dev_get_drvdata(dev);
>>> +	q->dev = dev;
>>
>> Why store the device with the vfio_ap_queue object? Why not just return
>> the device. The caller can get the vfio_ap_queue from the device's
>> driver data. It seems the only reason for the 'dev' field is to
>> temporarily hold a ref to the device so it can be put later. Why not
>> just put the device.

After thinking about this further, I question whether we even need
this function if it is going to return 'struct device *'. In that case,
why not just call driver_find_device() when the device is needed?
If you want to keep the function, then the function needs only one 
statement:

	return driver_find_device(...).

> 
> Having looked at the remainder of the patches, I tend to agree that we
> don't really need the backlink; we walk the driver's list of devices in
> any case IIUC.
> 
> We *might* want a mechanism to grab the queue quickly (i.e. without
> walking the list) if there's anything performance sensitive in there;
> but from the patch descriptions, I don't think anything is done in a
> hot path, so it should be fine.
>

Patch

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 900b9cf..2a52c9b 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -24,6 +24,60 @@ 
 #define VFIO_AP_MDEV_TYPE_HWVIRT "passthrough"
 #define VFIO_AP_MDEV_NAME_HWVIRT "VFIO AP Passthrough Device"
 
+/**
+ * vfio_ap_check_apqn: check if a ap_queue is of a given APQN
+ *
+ * Returns 1 if we have a match.
+ * Otherwise returns 0.
+ */
+static int vfio_ap_check_apqn(struct device *dev, void *data)
+{
+	struct vfio_ap_queue *q = dev_get_drvdata(dev);
+
+	return (q->apqn == *(int *)data);
+}
+
+/**
+ * vfio_ap_get_queue: Retrieve a queue with a specific APQN
+ * @apqn: The queue APQN
+ *
+ * Retrieve a queue with a specific APQN from the list of the
+ * devices associated to the vfio_ap_driver.
+ *
+ * The vfio_ap_queue has been already associated with the device
+ * during the probe.
+ * Store the associated device for reference counting
+ *
+ * Returns the pointer to the associated vfio_ap_queue
+ */
+static  __attribute__((unused))
+	struct vfio_ap_queue *vfio_ap_get_queue(int apqn)
+{
+	struct device *dev;
+	struct vfio_ap_queue *q;
+
+	dev = driver_find_device(&matrix_dev->vfio_ap_drv->driver, NULL, &apqn,
+				 vfio_ap_check_apqn);
+	if (!dev)
+		return NULL;
+	q = dev_get_drvdata(dev);
+	q->dev = dev;
+	return q;
+}
+
+/**
+ * vfio_ap_put_queue: lower device reference count for a queue
+ * @q: The queue
+ *
+ * put the associated device
+ *
+ */
+static  __attribute__((unused)) void vfio_ap_put_queue(struct vfio_ap_queue *q)
+{
+	put_device(q->dev);
+	q->dev = NULL;
+}
+
 static void vfio_ap_matrix_init(struct ap_config_info *info,
 				struct ap_matrix *matrix)
 {
diff --git a/drivers/s390/crypto/vfio_ap_private.h b/drivers/s390/crypto/vfio_ap_private.h
index 8836f01..081f0d7 100644
--- a/drivers/s390/crypto/vfio_ap_private.h
+++ b/drivers/s390/crypto/vfio_ap_private.h
@@ -87,6 +87,7 @@  extern int vfio_ap_mdev_register(void);
 extern void vfio_ap_mdev_unregister(void);
 
 struct vfio_ap_queue {
+	struct device *dev;
 	int	apqn;
 };
 #endif /* _VFIO_AP_PRIVATE_H_ */