linux-kernel - Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

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Date:	Wed, 18 Feb 2009 09:14:51 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Yinghai Lu <yinghai@...nel.org>, Nick Piggin <npiggin@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
 ipi fallback for smp_call_function_many())



On Wed, 18 Feb 2009, Ingo Molnar wrote:
>
> an off-list comment pointed out this piece of information as 
> well:
> 
>   http://www.sandpile.org/ia32/coherent.htm
> 
>   A WRMSR to one of the x2APIC MSRs (0000_0800h...0000_0BFFh) is 
>   not guaranteed to be serializing.
> 
> So i suspect we should just enclose it in smp_mb() pairs to make 
> sure it's a full barrier in both directions?

Why would we care about "both directions"?

I think putting an sfence _before_ the wrmsr (and not even all of them - 
just put it in front of the "send IPI" sequence) should be fine. Any other 
ordering sounds like just unnecessary overhead to me.

We do want this to be low-overhead, even if we probably don't care _that_ 
much.

			Linus
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