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

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Date:	Thu, 21 Apr 2016 16:30:07 +0200
From:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, james.hogan@...tec.com,
	mingo@...hat.com, linux-mips@...ux-mips.org, kvm@...r.kernel.org,
	rkrcmar@...hat.com, linux-kernel@...r.kernel.org,
	qemu-ppc@...gnu.org, Cornelia Huck <cornelia.huck@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH v4 1/2] KVM: remove NULL return path for vcpu ids >=
 KVM_MAX_VCPUS

On Thu, 21 Apr 2016 16:17:29 +0200
David Hildenbrand <dahi@...ux.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@...ux.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@...ux.vnet.ibm.com>
> 
> David

Thanks for the review !

Cheers.

--
Greg