linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Message-ID: <48752BAC.2050905@goop.org>
Date:	Wed, 09 Jul 2008 14:20:44 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Eric W. Biederman wrote:
> But we can pick an arbitrary point where %gs points at.
>
> Hmm.  This whole thing is even sillier then I thought.
> Why can't we access per cpu vars as:
> %gs:(per_cpu__var - __per_cpu_start) ?
>   

Because there's no linker reloc for doing subtraction (or addition) of 
two symbols.

> If we can subtract constants and allow the linker to perform that resolution
> at link.  A zero based per cpu segment becomes a moot issue.
>   

They're not constants; they're symbols.

    J
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