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

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Date:	Wed, 16 May 2012 20:21:55 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, x86@...nel.org,
	kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 3/5] kvm: host side for eoi optimization

On Wed, May 16, 2012 at 02:04:27PM -0300, Marcelo Tosatti wrote:
> On Wed, May 16, 2012 at 07:50:48PM +0300, Michael S. Tsirkin wrote:
> > 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.
> 
> Please describe the scenario clearly, i can't see the problem.

During vcpu entry there are two IRRs set 100 and 200. 200 is injected,
but irq window is not requested because 100 can't be injected, During
EOI exit 100 is injected.

--
			Gleb.
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