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

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Date:	Thu, 21 May 2009 18:26:38 +0200
From:	Borislav Petkov <borislav.petkov@....com>
To:	"H. Peter Anvin" <hpa@...or.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

On Thu, May 21, 2009 at 07:21:21AM -0700, H. Peter Anvin wrote:
> Borislav Petkov wrote:
> >>>
> >> smp_call_function_many() does allow concurrent execution.
> > 
> > Well, IMHO, not an absolutely concurrent execution since you can't
> > control at what moment in time the IPIs will be executed on each core,
> > IOW, the respective call function IPIs will generally not coincide on
> > each core at a given moment in time. This is especially true if you're
> > sending IPIs to cores across nodes.
> > 
> > And there's hypothetically a small window where you might get an
> > inconsistent MSR content across cores. It's a whole another question
> > whether this is going to be relevant.
> > 
> 
> It *allows* concurrent execution.  It doesn't *guarantee* concurrent
> execution.

Exactly what I'm trying to say.

> > So, the actual question is do we need that done in msr.c? If there are
> > no potential users, I could easily do the whole thing in the driver and
> > do not touch x86 code.
> 
> Only if we care about the additional performance of having it be
> nonserializing.

Well, let's do the nonserializing thing and have it done right from the
beginning. We might not care now but who knows..?

-- 
Regards/Gruss,
Boris.

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