linux-kernel - Re: [x86] do_arch

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Message-Id: <200812080002.21192.goretux@gmail.com>
Date:	Mon, 8 Dec 2008 00:02:20 +0100
From:	Eric Lacombe <goretux@...il.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [x86] do_arch_prctl

Hi,

I'm sorry to insist, but I really want to understand what occurs in this 
portion of kernel code. And that's why I resend my previous message with the 
hope that someone could enlighten my mind.

Thanks in advance,

	Eric

Le lundi 24 novembre 2008 19:22:18 Jeremy Fitzhardinge, vous avez écrit :
> Eric Lacombe wrote:
> > Hello,
> >
> > Does the "doit case" (line 822 in ARCH_GET_FS, function do_arch_prctl)
> > exist for performance reasons? Else, why "task->thread.fs" (line 824)
> > does not contain the fs base in the "doit case"?
>
> "doit" gets set when you're operating on yourself.  If you're operating
> on another process, then you need to use their task structure values
> rather than the current process's values.  If you're doing it to
> yourself, then the task structure may be out of date because its only
> updated on a context switch.

The task_struct is also updated in sys_arch_prctl (ARCH_SET_FS and 
ARCH_SET_GS), so not just on a context switch.
How the task structure could be out of date wrt thread.gs and thread.fs?
What could be a typical scenario that could induced gs or fs to be modified and 
not thread.gs and thread.fs?

Why we have a difference between ARCH_GET_GS :

> 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                 }

and ARCH_GET_FS :

> 821                 else if (doit)
> 822                         rdmsrl(MSR_FS_BASE, base);

If I follow what you say, why can't we have the same optimization in 
ARCH_GET_FS?

thanks,

	Eric



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