linux-kernel - Re: i386 PDA patches use of %gs

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Message-ID: <455B5FB6.7010009@goop.org>
Date:	Wed, 15 Nov 2006 10:43:02 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Arjan van de Ven <arjan@...radead.org>, akpm@...l.org, ak@...e.de,
	linux-kernel@...r.kernel.org, Michael.Fetterman@...cam.ac.uk,
	Ian Campbell <Ian.Campbell@...Source.com>
Subject: Re: i386 PDA patches use of %gs

Ingo Molnar wrote:
> for example, your test_fs() code does:
>
>         for(i = 0; i < COUNT; i++) {
>                 asm volatile("push %%fs; mov %1, %%fs; addl $1, %%fs:%0; popl %%fs"
>                              : "+m" (*offset): "r" (seg) : "memory");
>                 sync();
>         }
>
> that loads (and uses) a single selector value for %fs, and doesnt do any 
> mixed use as far as i can see.

I'm not sure what you're getting at.  Each loop iteration is analogous
to a user->kernel->user transition with respect to the
save/reload/use/restore pattern on the segment register.  In this case,
%fs starts as a null selector, gets reloaded with a non NULL selector,
and then is restored to null.  Do you mean some other mixing?

    J
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