Re: [PATCH] KVM: Avoid using vmx instruction directly

H. Peter Anvin
2006-11-21T18:35:48Z

Jeremy Fitzhardinge wrote:
> Avi Kivity wrote:
>>> Or gcc
>>> might move the assignment of phys_addr to after the inline assembly.
>>>
>> "asm volatile" prevents that (and I'm not 100% sure it's necessary).
>
> No, it won't necessarily. "asm volatile" simply forces gcc to emit the
> assembler, even if it thinks its output doesn't get used. It makes no
> ordering guarantees with respect to other code (or even other "asm
> volatiles"). The "memory" clobbers should fix the ordering of the asms
> though.
>

I think you're wrong about that; in particular, I'm pretty sure "asm
volatiles" are ordered among themselves. What the "volatile" means is
"this has side effects you (the compiler) don't understand", and gcc
can't assume that it can reorder such side effects.

-hpa

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