Re: [PATCH 03 of 36] x86: add memory barriers to wrmsr

[H. Peter Anvin]

Jeremy Fitzhardinge wrote:
> > Actually, I believe the barrier(); before is actually incorrect, since 
> > it would affect the wrmsr() register arguments rather than the wrmsr 
> > instruction itself.
>
> How so?  What kind of failure do think might occur?  Some effect on how 
> the wrmsr arguments are evaluated?
>
> barrier() is specifically a compiler optimisation barrier, so the 
> barrier before would prevent the compiler from moving anything logically 
> before the wrmsr to afterwards.

The barrier() before prevents the compiler from optimizing the access to 
the arguments (before they go into registers), not the actual wrmsr; 
this has to do with the ordering of operations around the barrier above. 
 The barrier *after* does what you just describe.

> That said, making the wrmsr itself a memory clobber may be simpler 
> understand with a comment, rather than separate barriers...

This should be functionally equivalent to a barrier(); after, and given 
that this is clearly a point of confusion *already*, I think the memory 
clobber is better.

	-hpa
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