linux-kernel - i386 PDA patches use of %gs

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Message-Id: <1158046540.2992.5.camel@laptopd505.fenrus.org>
Date:	Tue, 12 Sep 2006 09:35:40 +0200
From:	Arjan van de Ven <arjan@...radead.org>
To:	akpm@...l.org, ak@...e.de, mingo@...e.hu,
	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	linux-kernel@...r.kernel.org
Subject: i386 PDA patches use of %gs

Hi,

Userspace uses %gs for it's per thread data (and in modern linux
versions that means "all the time", errno is there for example).

On x86-64 this is the reason that the kernel uses the OTHER segment
register; so for the PDA patches this would mean using %fs and not %gs.

The advantage of this is very simple: %fs will be 0 for userspace most
of the time. Putting 0 in a segment register is cheap for the cpu,
putting anything else in is quite expensive (a LOT of security checks
need to happen). As such I would MUCH rather see that the i386 PDA
patches use %fs and not %gs... 

Jeremy, is there a reason you're specifically using %gs and not %fs? If
not, would you mind a switch to using %fs instead?

Greetings,
   Arjan van de Ven
-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com

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