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

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Message-ID: <Pine.LNX.4.64.0710161221300.1315@artax.karlin.mff.cuni.cz>
Date:	Tue, 16 Oct 2007 12:33:54 +0200 (CEST)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Nick Piggin <npiggin@...e.de>
cc:	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: LFENCE instruction (was: [rfc][patch 3/3] x86: optimise barriers)



On Tue, 16 Oct 2007, Nick Piggin wrote:

> > > The cpus also have an explicit set of instructions that deliberately do 
> > > unordered stores/loads, and s/lfence etc are mostly designed for those.
> > 
> > I know about unordered stores (movnti & similar) --- they basically use 
> > write-combining method on memory that is normally write-back --- and they 
> > need sfence. But which one instruction does unordered load and needs 
> > lefence?
> 
> Also, for non-wb memory. I don't think the Intel document referenced
> says anything about this, but the AMD document says that loads can pass
> loads (page 8, rule b).
> 
> This is why our rmb() is still an lfence.

I see, AMD says that WC memory loads can be out-of-order.

There is very little usability to it --- framebuffer and AGP aperture is 
the only piece of memory that is WC and no kernel structures are placed 
there, so it is possible to remove that lfence.

Mikulas
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