linux-kernel - Re: [x86] do_arch

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Date:	Mon, 08 Dec 2008 11:10:15 -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>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [x86] do_arch_prctl

Eric Lacombe wrote:
> 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.
>   

Well, its quite possible there are no good answers beyond "it needs a 
cleanup". 
> 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 yourself, then the task structure may be out of date because its only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 How the task structure could be out of date wrt thread.gs and thread.fs?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?
>   

Not sure.  It could just be redundant.

> 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?
>   

I haven't looked into it very closely, but its possible the asymmetry 
comes from the fact that there's no swapfs, and so the kernel and 
userspace aren't sharing %fs.

    J
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