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

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Date:	Wed, 09 Jul 2008 11:26:05 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Mike Travis <travis@....com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jack Steiner <steiner@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Christoph Lameter wrote:
> That is going to be difficult. The GS register is tied up for the pda area
> as long as you have it. And you cannot get rid of the pda because of the library
> compatibility issues. We would break binary compatibility if we would get rid of the pda.
>
> If one attempts to remove one field after another then the converted accesses will not be able to use GS relative accesses anymore. This can lead to all sorts of complications.
>   

Eh?  Yes they will.  That's the whole point of making the pda a percpu 
variable itself.  You can use %gs:<small> to get to the pda, and 
%gs:<larger> to get to percpu variables.  Converting pda->percpu will 
just have the effect of increasing the %gs offset into the percpu space.

This project isn't interesting to me unless per-cpu variables are 
directly accessible off %gs.

    J
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