linux-kernel - Re: [x86] do_arch_prctl

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Message-ID: <20081118064556.6b7dc245@infradead.org>
Date:	Tue, 18 Nov 2008 06:45:56 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Eric Lacombe <goretux@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [x86] do_arch_prctl - bug?

On Tue, 18 Nov 2008 15:33:32 +0100
Eric Lacombe <goretux@...il.com> wrote:

> Hello,
> 
> I would like to know why the ARCH_SET_GS action of sys_arch_prctl,
> write the MSR MSR_KERNEL_GS_BASE and not the MSR MSR_GS_BASE when the
> variable "doit" equals 1? Is that a bug?
> 

I don't think it is.
The trick is that we use "swapgs" on entering/leaving the kernel, and
that will "swap" gs with the MSR, so when we return to userspace, GS
gets loaded from the MSR_KERNEL_GS_BASE ...

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
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