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

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Date:	Wed, 16 May 2012 19:32:06 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Marcelo Tosatti <mtosatti@...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 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.

> 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.
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