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

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Date:	Wed, 15 Nov 2006 19:26:14 +0100
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:

> Ingo Molnar wrote:
> >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)
> 
> But it doesn't mix different descriptors on the same selector; the GDT 
> is initialized when the CPU is brought up, and is unchanged from then 
> on.  The PDA descriptor is GDT entry 27 and the userspace TLS entries 
> are 6-8, so in the typical case %gs will alternate between 0x33 and 
> 0xd8 as it enters and leaves the kernel.
> 
> My test program does the same thing, except using GDT entries 6 and 7 
> (selectors 0x33 and 0x3b).

no, that's not what it does. It measures 50000000 switches of the _same_ 
selector value, without using any of the selectors in the loop itself. 
I.e. no mixing at all! But when the kernel and userspace uses %gs, it's 
the cost of switching between two selector values of %gs that has to be 
measured. Your code does not measure that at all, AFAICS.

	Ingo
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