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

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Message-Id: <1213032042.3508.54.camel@localhost.localdomain>
Date:	Mon, 09 Jun 2008 12:20:41 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Glauber Costa <gcosta@...hat.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

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.

James


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