linux-kernel - Re: [PATCH 8/8] Convert PDA into the percpu section

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Date:	Tue, 06 Mar 2007 16:35:06 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Ingo Molnar <mingo@...e.hu>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>,
	Jeremy Fitzhardinge <jeremy@...source.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: [PATCH 8/8] Convert PDA into the percpu section

Rusty Russell wrote:
> 	If we used __thread, then gcc could do this optimization for us when it
> knows an rvalue is needed, however:
>
> 1) gcc wants to use %gs, not %fs, which is measurably slower for the
> kernel,
> 2) gcc wants to use huge offsets to store the address of the per-cpu
> space, and this breaks Xen (and current lguest, but new lguest no longer
> uses segments for protection)
>   

Well, if we go to the effort of teaching gcc how to use %fs, we can
probably convince it to generate positive offset TLS relocs too.

    J
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