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

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Date:	Mon, 11 Sep 2006 09:36:49 +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:
> >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.
> >  
> 
> Well, that means the only operation which would be different would be 
> the pop %fs at the end, since only it would end up loading a null 
> selector.  All the other operations would presumably take just as 
> long.

yes - but loading a null selector is a special-case: you dont have to 
invalidate/reload the shadow, you just have to turn access off. This 
might or might not make a difference on modern CPUs (it makes a 
difference with older CPUs) - but it's worth a try nevertheless. You 
measured a 9 cycles degradation with the %gs method, we could recover 
some of that.

	Ingo
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