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

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  [PATCH v4 1/2] KVM: remove NULL return path for vcpu ids >=  KVM_MAX_VCPUS Greg Kurz <gkurz@linux.vnet.ibm.com> - 2016-04-21 16:20 +0200
    Re: [PATCH v4 1/2] KVM: remove NULL return path for vcpu ids >=  KVM_MAX_VCPUS Cornelia Huck <cornelia.huck@de.ibm.com> - 2016-04-26 09:50 +0200
    Re: [PATCH v4 1/2] KVM: remove NULL return path for vcpu ids >=  KVM_MAX_VCPUS Gerg Kurz <gkurz@linux.vnet.ibm.com> - 2016-04-27 11:50 +0200
      Re: [PATCH v4 1/2] KVM: remove NULL return path for vcpu ids >=  KVM_MAX_VCPUS Radim Krčmář <rkrcmar@redhat.com> - 2016-04-27 16:50 +0200

#1384280 — [PATCH v4 1/2] KVM: remove NULL return path for vcpu ids >= KVM_MAX_VCPUS

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

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#1387196

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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#1388852

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)
>
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

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#1389169

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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