linux-kernel - Re: 2.6.18-rc6-mm1: GPF loop on early boot

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Message-ID: <20060911072959.GA2322@elte.hu>
Date:	Mon, 11 Sep 2006 09:29:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>,
	Laurent Riffard <laurent.riffard@...e.fr>,
	Arjan van de Ven <arjan@...radead.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...source.com>
Subject: Re: 2.6.18-rc6-mm1: GPF loop on early boot


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Ingo Molnar wrote:
> >another thing about i386-pda: why did you pick the %gs selector to store 
> >the PDA in? %fs would be a better choice because %gs is used by glibc so 
> >the saving/restoring of %fs would likely be near zero-cycles cost. 
> >(instead of the current 9 cycles for saving/restoring %gs)
> 
> Why would saving/restoring %fs be quicker? [...]

because userspace does not use it normally, while with %gs we'd switch 
between glibc's descriptor [which must be shadowed by the CPU] and the 
kernel's descriptor [which must be shadowed by the CPU too] - hence 
causing a constant reloading of the shadow register.

	Ingo
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