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

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Message-ID: <4FA93B3F.3000906@redhat.com>
Date:	Tue, 08 May 2012 17:26:55 +0200
From:	Paolo Bonzini <pbonzini@...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>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, gleb@...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

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.

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;

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