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

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Date:	Tue, 8 May 2012 18:28:43 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...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>,
	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 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.

> Paolo
> 
> > +static inline int kvm_test_and_clear_bit(int nr, volatile u16* addr)
> > +{
> > +	int oldbit;
> > +
> > +	asm volatile("btr %2,%1\n\t"
> > +		     "sbb %0,%0"
> > +		     : "=r" (oldbit), BITOP_ADDR_CONSTRAINT(*addr) : "Ir" (nr));
> > +	return oldbit;

--
			Gleb.
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