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    SubjectRe: 2.6.18-rc6-mm1: GPF loop on early boot

    * Jeremy Fitzhardinge <jeremy@goop.org> wrote:

    > Ingo Molnar wrote:
    > >another thing about i386-pda: why did you pick the %gs selector to store
    > >the PDA in? %fs would be a better choice because %gs is used by glibc so
    > >the saving/restoring of %fs would likely be near zero-cycles cost.
    > >(instead of the current 9 cycles for saving/restoring %gs)
    >
    > Why would saving/restoring %fs be quicker? [...]

    because userspace does not use it normally, while with %gs we'd switch
    between glibc's descriptor [which must be shadowed by the CPU] and the
    kernel's descriptor [which must be shadowed by the CPU too] - hence
    causing a constant reloading of the shadow register.

    Ingo
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