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

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Message-ID: <20060913095942.GA10075@elte.hu>
Date:	Wed, 13 Sep 2006 11:59:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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


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

> [...]  The basic inner loop is:
> 
> 	push %segreg
> 	mov  %selectorreg, %segreg
> 	add  $1,%segreg:offset	# use the segment register
> 	pop  %segreg

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)

	Ingo
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