* Jeremy Fitzhardinge <[email protected]> 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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