Re: [PART1 RFC v3 10/12] svm: Do not expose x2APIC when enable AVIC

[Suravee Suthikulpanit]

Hi Paolo,

On 3/31/16 18:23, Paolo Bonzini wrote:
> On 31/03/2016 06:15, Suravee Suthikulpanit wrote:
> > > > +                vcpu->arch.cpuid_entries[i].ecx &= ~(1 << 21);
> > >
> > > and X86_FEATURE_X2APIC (or something with X2APIC in name) for the
> > > bit.
> > >
> > > The code will become so obvious that the comment can be removed.
> > > :)
> >
> > Good point. I can only find example of using (X86_FEATURE_X2APIC %
> > 32) == 21.
>
> You can use bit(X86_FEATURE_X2APIC), it is defined in arch/x86/kvm/x86.h.

Ahh, thanks.

> > > but the MSR interface is going to exit and host-side interrupt
> > > delivery will probably still work, so I don't see a huge problem
> > > with it.
> >
> > Agree that it will still work. However, in such case, the guest code
> > would likely default to using x2APIC interface, which will not be
> > handled by the AVIC hardware, and resulting in no performance
> > improvement that we are trying to introduce.
>
> You would still get some improvement from exit-free interrupt delivery.

Let me look into this and investigate some more.

Thanks,
Suravee