diff mbox

[v4,1/2] KVM: remove NULL return path for vcpu ids >= KVM_MAX_VCPUS

Message ID 146124810201.32509.2946887043729554992.stgit@bahia.huguette.org (mailing list archive)
State New, archived
Headers show

Commit Message

Greg Kurz April 21, 2016, 2:15 p.m. UTC 
Commit c896939f7cff ("KVM: use heuristic for fast VCPU lookup by id") added
a return path that prevents vcpu ids to exceed KVM_MAX_VCPUS. This is a
problem for powerpc where vcpu ids can grow up to 8*KVM_MAX_VCPUS.

This patch simply reverses the logic so that we only try fast path if the
vcpu id can be tried as an index in kvm->vcpus[]. The slow path is not
affected by the change.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
---
 include/linux/kvm_host.h |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)


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Comments

David Hildenbrand April 21, 2016, 2:17 p.m. UTC | #1 
> Commit c896939f7cff ("KVM: use heuristic for fast VCPU lookup by id") added
> a return path that prevents vcpu ids to exceed KVM_MAX_VCPUS. This is a
> problem for powerpc where vcpu ids can grow up to 8*KVM_MAX_VCPUS.
> 
> This patch simply reverses the logic so that we only try fast path if the
> vcpu id can be tried as an index in kvm->vcpus[]. The slow path is not
> affected by the change.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
>  include/linux/kvm_host.h |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 5276fe0916fc..23bfe1bd159c 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -447,12 +447,13 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
> 
>  static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
>  {
> -	struct kvm_vcpu *vcpu;
> +	struct kvm_vcpu *vcpu = NULL;
>  	int i;
> 
> -	if (id < 0 || id >= KVM_MAX_VCPUS)
> +	if (id < 0)
>  		return NULL;
> -	vcpu = kvm_get_vcpu(kvm, id);
> +	if (id < KVM_MAX_VCPUS)
> +		vcpu = kvm_get_vcpu(kvm, id);

Maybe this check even should go into kvm_get_vcpu()

>  	if (vcpu && vcpu->vcpu_id == id)
>  		return vcpu;
>  	kvm_for_each_vcpu(i, vcpu, kvm)
> 

Anyhow,

Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>

David

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Greg Kurz April 21, 2016, 2:30 p.m. UTC | #2 
On Thu, 21 Apr 2016 16:17:29 +0200
David Hildenbrand <dahi@linux.vnet.ibm.com> wrote:

> > Commit c896939f7cff ("KVM: use heuristic for fast VCPU lookup by id") added
> > a return path that prevents vcpu ids to exceed KVM_MAX_VCPUS. This is a
> > problem for powerpc where vcpu ids can grow up to 8*KVM_MAX_VCPUS.
> > 
> > This patch simply reverses the logic so that we only try fast path if the
> > vcpu id can be tried as an index in kvm->vcpus[]. The slow path is not
> > affected by the change.
> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> >  include/linux/kvm_host.h |    7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 5276fe0916fc..23bfe1bd159c 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -447,12 +447,13 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
> > 
> >  static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
> >  {
> > -	struct kvm_vcpu *vcpu;
> > +	struct kvm_vcpu *vcpu = NULL;
> >  	int i;
> > 
> > -	if (id < 0 || id >= KVM_MAX_VCPUS)
> > +	if (id < 0)
> >  		return NULL;
> > -	vcpu = kvm_get_vcpu(kvm, id);
> > +	if (id < KVM_MAX_VCPUS)
> > +		vcpu = kvm_get_vcpu(kvm, id);  
> 
> Maybe this check even should go into kvm_get_vcpu()
> 

Yeah possibly, but there are 19 users for kvm_get_vcpu() and I'm not sure if
none of them is on a hot path where this extra check could hurt... maybe this
can be done in a cleanup patch afterwards ?

> >  	if (vcpu && vcpu->vcpu_id == id)
> >  		return vcpu;
> >  	kvm_for_each_vcpu(i, vcpu, kvm)
> >   
> 
> Anyhow,
> 
> Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
> 
> David

Thanks for the review !

Cheers.

--
Greg

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Cornelia Huck April 26, 2016, 7:44 a.m. UTC | #3 
On Thu, 21 Apr 2016 16:15:05 +0200
Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:

> Commit c896939f7cff ("KVM: use heuristic for fast VCPU lookup by id") added
> a return path that prevents vcpu ids to exceed KVM_MAX_VCPUS. This is a
> problem for powerpc where vcpu ids can grow up to 8*KVM_MAX_VCPUS.
> 
> This patch simply reverses the logic so that we only try fast path if the
> vcpu id can be tried as an index in kvm->vcpus[]. The slow path is not
> affected by the change.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
>  include/linux/kvm_host.h |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

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Greg Kurz April 27, 2016, 9:40 a.m. UTC | #4 
Quoting Greg Kurz <gkurz@linux.vnet.ibm.com>:

> Commit c896939f7cff ("KVM: use heuristic for fast VCPU lookup by id") added
> a return path that prevents vcpu ids to exceed KVM_MAX_VCPUS. This is a
> problem for powerpc where vcpu ids can grow up to 8*KVM_MAX_VCPUS.
>
> This patch simply reverses the logic so that we only try fast path if the
> vcpu id can be tried as an index in kvm->vcpus[]. The slow path is not
> affected by the change.
>
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---

Radim,

I think this sanity check is only needed because kvm_get_vcpu() use the
id as an index in kvm->vcpus[]. Checking against the new KVM_MAX_VCPU_ID
would be clearly wrong here.

And this patch got two R-b tags already. Do you agree we keep it ?

Cheers.

--
Greg

>  include/linux/kvm_host.h |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 5276fe0916fc..23bfe1bd159c 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -447,12 +447,13 @@ static inline struct kvm_vcpu  
> *kvm_get_vcpu(struct kvm *kvm, int i)
>
>  static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
>  {
> -	struct kvm_vcpu *vcpu;
> +	struct kvm_vcpu *vcpu = NULL;
>  	int i;
>
> -	if (id < 0 || id >= KVM_MAX_VCPUS)
> +	if (id < 0)
>  		return NULL;
> -	vcpu = kvm_get_vcpu(kvm, id);
> +	if (id < KVM_MAX_VCPUS)
> +		vcpu = kvm_get_vcpu(kvm, id);
>  	if (vcpu && vcpu->vcpu_id == id)
>  		return vcpu;
>  	kvm_for_each_vcpu(i, vcpu, kvm)
>
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Radim Krčmář April 27, 2016, 2:40 p.m. UTC | #5 
2016-04-27 05:40-0400, Gerg Kurz:
> Quoting Greg Kurz <gkurz@linux.vnet.ibm.com>:
> 
>> Commit c896939f7cff ("KVM: use heuristic for fast VCPU lookup by id") added
>> a return path that prevents vcpu ids to exceed KVM_MAX_VCPUS. This is a
>> problem for powerpc where vcpu ids can grow up to 8*KVM_MAX_VCPUS.
>> 
>> This patch simply reverses the logic so that we only try fast path if the
>> vcpu id can be tried as an index in kvm->vcpus[]. The slow path is not
>> affected by the change.
>> 
>> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
>> ---
> 
> Radim,
> 
> I think this sanity check is only needed because kvm_get_vcpu() use the
> id as an index in kvm->vcpus[]. Checking against the new KVM_MAX_VCPU_ID
> would be clearly wrong here.

I agree, checking KVM_MAX_VCPU_ID would be pointless.

> And this patch got two R-b tags already. Do you agree we keep it ?

Yes, thanks.
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diff mbox

Patch

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 5276fe0916fc..23bfe1bd159c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -447,12 +447,13 @@  static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
 
 static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
 {
-	struct kvm_vcpu *vcpu;
+	struct kvm_vcpu *vcpu = NULL;
 	int i;
 
-	if (id < 0 || id >= KVM_MAX_VCPUS)
+	if (id < 0)
 		return NULL;
-	vcpu = kvm_get_vcpu(kvm, id);
+	if (id < KVM_MAX_VCPUS)
+		vcpu = kvm_get_vcpu(kvm, id);
 	if (vcpu && vcpu->vcpu_id == id)
 		return vcpu;
 	kvm_for_each_vcpu(i, vcpu, kvm)