linux-kernel - Re: LFENCE instruction (was: [rfc][patch 3/3] x86: optimise barriers)

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Message-ID: <20071015143732.01d99af8@laptopd505.fenrus.org>
Date:	Mon, 15 Oct 2007 14:37:32 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: LFENCE instruction (was: [rfc][patch 3/3] x86: optimise
 barriers)

On Mon, 15 Oct 2007 22:47:42 +0200 (CEST)
Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz> wrote:

> > According to latest memory ordering specification documents from
> > Intel and AMD, both manufacturers are committed to in-order loads
> > from cacheable memory for the x86 architecture. Hence, smp_rmb()
> > may be a simple barrier.
> >
> > http://developer.intel.com/products/processor/manuals/318147.pdf 
> > http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/24593.pdf
> 
> Hi
> 
> I'm just wondering about one thing --- what is LFENCE instruction
> good for?
> 
> SFENCE is for enforcing ordering in write-combining buffers (it
> doesn't have sense in write-back cache mode).
> MFENCE is for preventing of moving stores past loads.
> 
> But what is LFENCE for? I read the above documents and they already
> say that CPUs have ordered loads.
> 

The cpus also have an explicit set of instructions that deliberately do
unordered stores/loads, and s/lfence etc are mostly designed for those.
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