Message-ID: <48752BAC.2050905@goop.org>
Date: Wed, 09 Jul 2008 14:20:44 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: "Eric W. Biederman" <ebiederm@xmission.com>
CC: "H. Peter Anvin" <hpa@zytor.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Ingo Molnar <mingo@elte.hu>, Mike Travis <travis@sgi.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Jack Steiner <steiner@sgi.com>, linux-kernel@vger.kernel.org,
       Arjan van de Ven <arjan@infradead.org>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses
References: <20080709165129.292635000@polaris-admin.engr.sgi.com>	<m1abgqzv3g.fsf@frodo.ebiederm.org> <20080709200757.GD14009@elte.hu>	<48751B57.8030605@goop.org> <48751CF9.4020901@linux-foundation.org>	<4875209D.8010603@goop.org> <487522A8.60906@zytor.com>	<48752409.1030201@goop.org> <m1abgqu5rg.fsf@frodo.ebiederm.org>
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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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