linux-kernel - Re: [rfc 37/45] x86_64: Support for fast per cpu operations

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Message-Id: <20071119.181720.03409715.davem@davemloft.net>
Date:	Mon, 19 Nov 2007 18:17:20 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	hpa@...or.com
Cc:	clameter@....com, ak@...e.de, akpm@...ux-foundation.org,
	travis@....com, mathieu.desnoyers@...ymtl.ca,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc 37/45] x86_64: Support for fast per cpu operations

From: "H. Peter Anvin" <hpa@...or.com>
Date: Mon, 19 Nov 2007 18:00:23 -0800

> Christoph Lameter wrote:
> > Support fast cpu ops in x86_64 by providing a series of functions that
> > generate the proper instructions. Define CONFIG_FAST_CPU_OPS so that core code
> > can exploit the availability of fast per cpu operations.
> > 
> > Signed-off-by: Christoph Lameter <clameter@....com>
> 
> There was, at some point, discussion about using the gcc TLS mechanism, 
> which should permit even better code to be generated.  Unfortunately, it 
> would require gcc to be able to reference %gs instead of %fs (and vice 
> versa for i386), which I don't think is available in anything except 
> maybe the most cutting-edge version of gcc.

You can't use __thread because GCC will cache __thread computed
addresses across context switches and cpu changes.

It's been tried before on powerpc, it doesn't work.
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