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

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Date:	Wed, 16 May 2012 21:38:22 +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 09:29:49PM +0300, Michael S. Tsirkin wrote:
> On Wed, May 16, 2012 at 09:25:20PM +0300, Gleb Natapov wrote:
> > On Wed, May 16, 2012 at 03:15:00PM -0300, Marcelo Tosatti wrote:
> > > On Wed, May 16, 2012 at 08:58:57PM +0300, Michael S. Tsirkin wrote:
> > > > 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.
> > > 
> > > Right, it is not redundant.
> > > 
> > > The above is still a bug: a case where eoi pending bit is not updated
> > > properly.
> > When IRR is set while eoi_skip is enabled, eoi_skip should be cleared.
> > Michael does your patch do that?
> 
> I think this does it:
>         /* Detect interrupt nesting and disable EOI optimization */
>         if (pv_eoi_enabled(vcpu) && vector == -2)
>                 pv_eoi_clr_pending(vcpu);
> 
> -2 is returned when irr is set.
> 
This code is reached from kvm_cpu_get_interrupt(), but this function will
not be called in above scenario.

> 
> > > 
> > > How come is this compatible with hyper-v again? Enlight me.
> > 
> > --
> > 			Gleb.

--
			Gleb.
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