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

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Date:	Wed, 13 Sep 2006 09:17:32 -0700
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:
> well, the most important thing i believe you didnt test: the effect of 
> mixing two descriptors on the _same_ selector: one %gs selector value 
> loaded and used by glibc, and another %gs selector value loaded and used 
> by the kernel, intermixed. It's the mixing that causes the descriptor 
> cache reload. (unless i missed some detail about your testcase)

But it doesn't mix different descriptors on the same selector; the GDT 
is initialized when the CPU is brought up, and is unchanged from then 
on.  The PDA descriptor is GDT entry 27 and the userspace TLS entries 
are 6-8, so in the typical case %gs will alternate between 0x33 and 0xd8 
as it enters and leaves the kernel.

My test program does the same thing, except using GDT entries 6 and 7 
(selectors 0x33 and 0x3b).

    J

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