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

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Date:	Wed, 15 Nov 2006 19:29:15 +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


* Ingo Molnar <mingo@...e.hu> wrote:

> > 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.

for example, your test_fs() code does:

        for(i = 0; i < COUNT; i++) {
                asm volatile("push %%fs; mov %1, %%fs; addl $1, %%fs:%0; popl %%fs"
                             : "+m" (*offset): "r" (seg) : "memory");
                sync();
        }

that loads (and uses) a single selector value for %fs, and doesnt do any 
mixed use as far as i can see.

	Ingo
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