Re: [PATCHv4 3/5] kvm: host side for eoi optimization

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 16, 2012 at 07:32:06PM +0300, Gleb Natapov wrote:
> On Wed, May 16, 2012 at 07:22:47PM +0300, Michael S. Tsirkin wrote:
> > On Wed, May 16, 2012 at 12:49:40PM -0300, Marcelo Tosatti wrote:
> > > > @@ -1245,9 +1306,20 @@ int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu)
> > > >  	int vector = kvm_apic_has_interrupt(vcpu);
> > > >  	struct kvm_lapic *apic = vcpu->arch.apic;
> > > >  
> > > > -	if (vector == -1)
> > > > +	/* Detect interrupt nesting and disable EOI optimization */
> > > > +	if (pv_eoi_enabled(vcpu) && vector == -2)
> > > > +		pv_eoi_clr_pending(vcpu);
> > > > +
> > > > +	if (vector < 0)
> > > 
> > > With interrupt window exiting, the guest will exit:
> > > 
> > > - as soon as it sets RFLAGS.IF=1 and there is any 
> > > interrupt pending in IRR.
> > > - any new interrupt is set in IRR will kick vcpu
> > > out of guest mode and recalculate interrupt-window-exiting.
> > > 
> > > Doesnt this make this bit unnecessary ?
> > 
> > Looks like we could cut it out.  But I'm not sure how architectural it is
> > that we exit on interrupt window.
> We request exit on interrupt window only if there is pending irq that
> can be delivered on a guest entry.

Aha. If so what Marcelo proposed won't work I think: if we inject A then B
which is lower priority, we need an exit on EOI, we can't inject
immediately.

> > I guess there are reasons to exit on interrupt window but
> > isn't it better to make the feature independent of it?
> > 
> > This almost never happens in my testing anyway, so
> > however we handle it is unlikely to affect performance.
> > 
> > -- 
> > MST
> 
> --
> 			Gleb.