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SubjectRe: [RESEND PATCH v6 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions
On Mon, 25 Mar 2019, Thomas Gleixner wrote:

> On Mon, 25 Mar 2019, Thomas Gleixner wrote:
> > The whole point of FSGSBASE support is performance, right?
> >
> > So can please someone explain why having the following in the context
> > switch path when it can be completely avoided is enhancing performance:
> >
> > - 4 x SWAPGS
> > - 1 x RDMSR
> > - 1 x WRMSR
>
> Corrrecting myself. That should be:
>
> RDGSBASE
> WRGSBASE
>
> obviously. Still the point remains.
>
> > - 2 x local_irq_save()
> > - 2 x local_irq_restore()
> >
> > Of course the local_irq_save/restore() pairs are utterly pointless because
> > switch_to() runs with interrupts disabled already.
> >
> > SWAPGS instead needs:
> >
> > 1 x WRMSR
> >
> > and nothing else.
> >
> > So trading the single WRMSR against the above in the context switch path is
> > gaining performance, right?

And even IF the sequences are faster than the single WRMSR, this does not
justify the mixed bag of SWAPGS/FSGSBASE usage at all.

Thanks,

tglx

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