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

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Date:	Wed, 15 Nov 2006 10:39:20 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
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 wrote:
> 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.
>   
I think you're misreading it.  This is the inner loop:

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

On entry, %gs will contain the normal usermode TLS selector.  "seg" is
another selector allocated with set_thread_area().  The asm pushes the
old %gs, loads the new one, uses a memory address via the new segment,
then restores the previous %gs.

So given this output:

"Genuine Intel(R) CPU           T2400  @ 1.83GHz" @1000Mhz (6,14,8):
ds=7b fs=0 gs=33 ldt=f gdt=3b CPUTIME 
[...]

The initial %fs and %gs are 0 and 0x33 respectively, and it is using
0x3b as the other GDT selector (and 0xf as the other LDT selector).

    J
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