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Date: Mon, 10 Apr 2017 14:53:39 +0200
From: Sahid Orentino Ferdjaoui <sferdjao@redhat.com>
Message-ID: <20170410125339.GA31761@redhat>
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Subject: Re: [Qemu-devel] [PATCH]Enable kvmvapic only when host doesn't
 support VAPIC capability in KVM mode
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To: "Xu, Anthony" <anthony.xu@intel.com>
Cc: "'qemu-devel@nongnu.org'" <qemu-devel@nongnu.org>, 'Paolo Bonzini' <pbonzini@redhat.com>, 'Stefan Hajnoczi' <stefanha@gmail.com>

On Thu, Apr 06, 2017 at 06:18:33PM +0000, Xu, Anthony wrote:
> > > --- a/kvm-all.c
> > > +++ b/kvm-all.c
> > > @@ -2232,6 +2232,10 @@ int kvm_has_sync_mmu(void)
> > >      return kvm_check_extension(kvm_state, KVM_CAP_SYNC_MMU);
> > >  }
> > >
> > > +int kvm_has_vapic(void){
> > > +    return !kvm_check_extension(kvm_state, KVM_CAP_VAPIC);
> > > +}
> > > +
> > 
> > Is that function shouldn't return true if KVM is providing VAPIC
> > capability?
> > 
> 
> It should, but it doesn't. see below KVM kernel code segment,
> it returns false if KVM provides VAPIC.

I think we shouldn't read it like that. It seems that KVM is always
returning the VAPIC capability except when the CPU is providing a
special acceleration [0].

I would say you can't really refer yourself at this bit to enable or
not kvmapic in QEMU.

Does that make sense?

[0] https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?id=774ead3ad9bcbc05ef6aaebb9bdf8b4c3126923b

> Thanks for your comments,
> 
> Will resend the patch.
> 
> 
> Anthony
> 
> 
> 
> arch/x86/kvm/x86.c
> 
>         case KVM_CAP_VAPIC:
>                 r = !kvm_x86_ops->cpu_has_accelerated_tpr();
>                 break;
>