linux-kernel - Re: Intel Memory Ordering White Paper

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Message-Id: <200709081053.36842.ak@suse.de>
Date:	Sat, 8 Sep 2007 10:53:36 +0200
From:	Andi Kleen <ak@...e.de>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jesse Barnes <jesse.barnes@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Intel Memory Ordering White Paper

On Friday 07 September 2007 20:13:12 Nick Piggin wrote:
> On Sunday 09 September 2007 03:48, Nick Piggin wrote:
> 
> > There is some suggestion in the source code that non-temporal stores
> > (movntq) are weakly ordered. But AFAIKS from the documents, it is ordered
> > when operating on wb memory. What's the situation there?
> 
> Sorry, it looks from the AMD document like nontemporal stores to wb
> memory can go out of order.

Yes, that is how NT stores are defined.
 
> If this is the case, we can either retain the sfence in smp_wmb(), or noop
> it, and put explicit sfences around any place that performs nontemporal
> stores...

We do this already, but in most cases it doesn't matter anyways. We AFAIK
do not rely on any ordering for copy_*_user for example. There are not
that many users of nt so it's not a huge issue.

> 
> Anyway, the lfence should be able to go away without so much trouble.

You mean sfence? lfence in rmb is definitely needed.

sfence on x86-64 is not strictly needed, but also shouldn't hurt very much 
so I always kept it in.

-Andi
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