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

Greg Kurz
2016-04-21T14:30:26Z

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