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

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Message-ID: <20120516162247.GA10676@redhat.com>
Date:	Wed, 16 May 2012 19:22:47 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	x86@...nel.org, kvm@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	gleb@...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 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.
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
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