linux-kernel - Re: [PATCH RFC dontapply 5/5] kvm_para: guest side for eoi avoidance

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Date:	Tue, 8 May 2012 19:32:14 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>, x86@...nel.org,
	kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC dontapply 5/5] kvm_para: guest side for eoi avoidance

On Tue, May 08, 2012 at 08:45:39AM -0700, H. Peter Anvin wrote:
> On 05/08/2012 08:28 AM, Gleb Natapov wrote:
> > On Tue, May 08, 2012 at 05:26:55PM +0200, Paolo Bonzini wrote:
> >> Il 23/04/2012 16:04, Michael S. Tsirkin ha scritto:
> >>> +/* Our own copy of __test_and_clear_bit to make sure
> >>> + * it is done with a single instruction */
> >>
> >> Is this for microoptimization or correctness?  If the latter, it does
> >> not ensure anything without a "lock" prefix.
> >>
> > It can't race with other vcpus, only with vmexit on the same vcpu.
> > 
> 
> That doesn't answer the question very well... I really don't understand
> the point of having a private copy here.
__test_and_clear_bit() is not guarantied to be local CPU atomic an that
is what we need here.

> 
> I really, really don't want a bunch of private interfaces around.  It
> would be a lot better to define a test_and_{set,clear}_bit_local() in
> <asm/bitops.h> which is defined to be local CPU atomic.
> 
Yes, this will be definitely better.

--
			Gleb.
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