linux-kernel - Re: [PATCH 01/22] x86: add methods for writing of an MSR on several CPUs

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Date:	Wed, 20 May 2009 14:39:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <borislav.petkov@....com>
CC:	akpm@...ux-foundation.org, greg@...ah.com, mingo@...e.hu,
	norsk5@...oo.com, tglx@...utronix.de, mchehab@...hat.com,
	aris@...hat.com, edt@....ca, linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [PATCH 01/22] x86: add methods for writing of an MSR on	several
 CPUs

Borislav Petkov wrote:
> 
> We currently need them for enabling the NB error reporting bank over
> MCG_CTL on each core on the node. The question is whether we really need
> the concurrency when accessing an MSR on several cores. With MCG_CTL,
> BKDG says "It is expected that this register is programmed to the same
> value in all nodes," but nothing concerning concurrency.
> 
> But you're right, if this interface is supposed to be generic enough,
> it is probably wise to access an MSR concurrently. I could imagine
> an obscure case where this is required. However, is sending IPIs
> (smp_call_function_many) guaranteeing the needed concurrency? Or, should
> it be more like how the mtrr code jumps through hoops (set_mtrr())
> in order to ensure that _ALL_ registers have been written _before_
> continuing?
> 

smp_call_function_many() does allow concurrent execution.  Looping over
a list with smp_call_function_one() -- which you currently have -- is
serializing, at which point we might just push the loop into the caller
rather than worrying about a new interface.

	-hpa

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