linux-kernel - Re: [PATCH v4 1/2] KVM: remove NULL return path for vcpu ids >= KVM_MAX

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Date:	Wed, 27 Apr 2016 05:40:52 -0400
From:	Gerg Kurz <gkurz@...ux.vnet.ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, james.hogan@...tec.com,
	mingo@...hat.com
Cc:	linux-mips@...ux-mips.org, kvm@...r.kernel.org, rkrcmar@...hat.com,
	linux-kernel@...r.kernel.org,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	qemu-ppc@...gnu.org, Cornelia Huck <cornelia.huck@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	David Gibson <david@...son.dropbear.id.au>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v4 1/2] KVM: remove NULL return path for vcpu ids >=
 KVM_MAX_VCPUS


Quoting Greg Kurz <gkurz@...ux.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@...ux.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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