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

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Date:	Thu, 17 May 2012 12:28:22 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	x86@...nel.org, kvm@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...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 05/16/2012 02:46 PM, Michael S. Tsirkin wrote:
> Implementation of PV EOI using shared memory.
> This reduces the number of exits an interrupt
> causes as much as by half.
>
> The idea is simple: there's a bit, per APIC, in guest memory,
> that tells the guest that it does not need EOI.
> We set it before injecting an interrupt and clear
> before injecting a nested one. Guest tests it using
> a test and clear operation - this is necessary
> so that host can detect interrupt nesting -
> and if set, it can skip the EOI MSR.
>
> There's a new MSR to set the address of said register
> in guest memory. Otherwise not much changed:
> - Guest EOI is not required
> - Register is tested & ISR is automatically cleared on exit
>
> For testing results see description of previous patch
> 'kvm_para: guest side for eoi avoidance'.
>
>  
> +	/*
> +	 * It's legal for guest to ignore the PV EOI optimization
> +	 * and signal EOI by APIC write. If this happens, clear
> +	 * PV EOI on guest's behalf.
> +	 */
> +	if (pv_eoi_enabled(apic->vcpu))
> +		pv_eoi_clr_pending(apic->vcpu);

I'm a little worried about all the clr_pending() calls scattered
around.  What happens if we forget one?  In particular, we might miss
one on nested vmentry.

A safer path is to always clear it, but to enable it again during
reentry if all conditions are satisified.  Might be a little slower though.

-- 
error compiling committee.c: too many arguments to function

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