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

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Date:	Wed, 25 Jun 2008 16:42:54 -0700
From:	"H. Peter Anvin" <hpa@...nel.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	xen-devel <xen-devel@...ts.xensource.com>,
	Stephen Tweedie <sct@...hat.com>,
	Eduardo Habkost <ehabkost@...hat.com>,
	Mark McLoughlin <markmc@...hat.com>
Subject: Re: [PATCH 03 of 36] x86: add memory barriers to wrmsr

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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