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

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Message-ID: <450510E0.8080906@goop.org>
Date:	Mon, 11 Sep 2006 00:31:44 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
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

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?  The main reason I chose %gs 
was so that it didn't add anything to the context switch time (since %gs 
needed to be switched anyway), and it leaves open the possibility of 
using gcc's TLS support in the kernel (I have a working demonstration of 
this, but Rusty and I are still working out the module details).

    J
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