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

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Date:	Tue, 20 Nov 2007 04:23:07 +0100
From:	Andi Kleen <ak@...e.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	David Miller <davem@...emloft.net>, clameter@....com,
	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

On Tuesday 20 November 2007 03:19, H. Peter Anvin wrote:
> David Miller wrote:
> >> 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.
>
> OK, that pretty much answers that question.

I investigated that some time ago.

There are other obstacles too on x86-64, e.g. the relocations
are wrong for kernel mode. You would need to extend the linker first.

-Andi
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