linux-kernel - Re: [PATCH 03/15] x86: remove early_gdt

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Message-ID: <484D670C.6050203@redhat.com>
Date:	Mon, 09 Jun 2008 14:23:24 -0300
From:	Glauber Costa <gcosta@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	tglx@...utronix.de, mingo@...e.hu, hugh@...itas.com
Subject: Re: [PATCH 03/15] x86: remove early_gdt_descr reference

James Bottomley wrote:
> On Mon, 2008-06-09 at 12:49 -0300, Glauber Costa wrote:
>> James Bottomley wrote:
>>> On Mon, 2008-06-09 at 11:16 -0300, Glauber Costa wrote:
>>>> since we use switch_to_new_gdt, there is no point
>>>> in assigning early_gdt_descr except for the first
>>>> assignment, which is done manually.
>>> What makes you think you can do this?  If you don't update the early
>>> boot gdt, they all end up using the Boot CPU one.  The problem with this
>>> is that there's a time from start_secondary to switch_to_new_gdt where
>>> the per cpu selector (%fs) and the pda selector (%gs) are those of the
>>> boot CPU.   The former isn't a problem but the CPU number is in the
>>> latter, and it's used in that path before we get to the initialisation.
>> You are right, I missed it.
>>
>> However, it only seem to be used in cpu_init, and very early. Sure there 
>> are some users _before_ we load the new gdt, but nothing prevents them 
>> to be moved after it. (Of course, this patch is wrong anyway).
>>
>> And if we do that, we can even take the %fs loading out of head_32.S
>> Of course, it's only valid if those are indeed the only early users of it.
>>
>> Is there any other use I'm missing?
> 
> Well, %fs loading there is done for the boot CPU. To eliminate that you
> have to not only verify that start_secondary doesn't use anything in
> per_cpu areas, but also verify that nothing in start_kernel() up until
> boot_cpu_init() does ... That's a lot of smp_processor_id() references
> to convert.
Yes, after a second look, it would be tricky indeed. But only for cpu0.
For all the others, I still think we could get rid of the problem by 
switching to the new gdt earlier in cpu_init.

What do you think?

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