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

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Date:	Wed, 25 Jun 2008 16:37:48 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...nel.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

H. Peter Anvin wrote:
> Arjan van de Ven wrote:
>>>>   
>>> I suppose, though I would be inclined to put the barriers in the
>>> wrmsr macro itself to act as documentation. 
>>
>>
>> yeah I meant like this:
>>
>> static inline void native_write_msr(unsigned int msr,
>>                      unsigned low, unsigned high)
>> {
>>     barrier();
>>     asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high));
>>     barrier();
>> }
>>
>> or in the same in the thing that calls this.
>>
>
> 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.

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

    J
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