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    SubjectRe: [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.


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