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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