linux-kernel - Re: [x86] do_arch_prctl

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Message-ID: <492366CB.8020905@goop.org>
Date:	Tue, 18 Nov 2008 17:07:23 -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 - bug?

Eric Lacombe wrote:
> I look at the Intel docs (vol. 3A) again, and see that in 64 bits mode the 
> hidden field gs.base are physically mapped to the MSR, so it seems that in 
> order to load gs.base we don't need to load gs (like in 32 bits mode), but 
> rather we only need to load the MSR.
>
> So I don't understand the purpose of load_gs_index in that context :
>
> if (doit) {
> 	load_gs_index(0);
> 	ret = checking_wrmsrl(MSR_KERNEL_GS_BASE, addr);
> }
>
> Why don't we only load the MSR ?
> What is the purpose of calling load_gs_index with 0 as parameter ?
>   

Because %gs of 0 means "base too large, go to MSR".  If you have a 
32-bit base, then loading it into the gdt and loading %gs with the right 
selector is faster.  wrmsr/rdmsr are slow instructions.

    J
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