linux-kernel - Re: [x86] do_arch

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Date:	Wed, 19 Nov 2008 16:07:42 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Eric Lacombe <goretux@...il.com>
CC:	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [x86] do_arch_prctl

Eric Lacombe wrote:
> Thanks for your answer, I've got one last question ;)
> In the ARCH_GET_GS, can you explain the line 834 to 838?
>
> In fact, at first sight I thought that just the line 836 was sufficient, but I 
> obviously miss the case where MSR_KERNEL_GS_BASE does not reflect the value 
> requested, hence my question.
>   

I think the rationale is that rdmsr is slow, so reading the value from 
the task context is faster where possible.

It's not a very pretty part of the architecture :/

    J

> 828 case ARCH_GET_GS: {
> 829                 unsigned long base;
> 830                 unsigned gsindex;
> 831                 if (task->thread.gsindex == GS_TLS_SEL)
> 832                         base = read_32bit_tls(task, GS_TLS);
> 833                 else if (doit) {
> 834                         asm("movl %%gs,%0" : "=r" (gsindex));
> 835                         if (gsindex)
> 836                                 rdmsrl(MSR_KERNEL_GS_BASE, base);
> 837                         else
> 838                                 base = task->thread.gs;
> 839                 }
> 840                 else
> 841                         base = task->thread.gs;
>
> Thanks,
>
> 	Eric
>
>
>   

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