* Jeremy Fitzhardinge <[email protected]> wrote:
> Arjan van de Ven wrote:
> > segment register accesses really are not cheap.
> > Also really it'll be better to use the register userspace is not using,
> > but we had that discussion before; could you remind me why you picked
> > %gs in the first place?
> >
>
> To leave open the possibility of using the compiler's TLS support in
> the kernel for percpu. I also measured the cost of reloading %gs vs
> %fs, and found no difference between reloading a null selector vs a
> non-null selector.
what point would there be in using it? It's not like the kernel could
make use of the thread keyword anytime soon (it would need /all/
architectures to support it) ... and the kernel doesnt mind how the
current per_cpu() primitives are implemented, via assembly or via C. In
any case, it very much matters to see the precise cost of having the pda
selector value in %gs versus %fs.
Ingo
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