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

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Date:	Wed, 16 May 2012 20:58:57 +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 02:20:58PM -0300, Marcelo Tosatti 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.
> > I guess there are reasons to exit on interrupt window but
> > isn't it better to make the feature independent of it?
> 
> Hum... not sure. Is it helpful for the Hyper-V interface?
> 
> > This almost never happens in my testing anyway, so
> > however we handle it is unlikely to affect performance.
> 
> It decreases the amount of state that must be maintained.
> 
> BTW there is a bug covered by interrupt window exiting:
> 
> vcpu0                               host
> - irr 5 set
> - isr 5 set, irr 5 cleared
> - eoi_skip bit not set, 
> no other bit set in irr.
> - enter guest
> 
>                                     irr 4 set
>                                     kick vcpu0 out of guest mode
> 
> - eoi pending bit not set
>   (previous interrupt injection 
>    still pending)
> - skip eoi
> 
> If it were not for interrupt window exiting, this would 
> inject vector 4 on an unrelated exit who knows how long 
> in the future.
> 
> Also note optimization depends on the fact that the host 
> kicks vcpu out unconditionally (so it is dependent on 
> certain kvm implementation details).
> 
> 
> 



Look we can summarize as follows: irq windows exit is
required both before and after this patch.
But it does not make the check above redundant.
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