lkml.org 
[lkml]   [2019]   [Mar]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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

\
 
 \ /
  Last update: 2019-03-25 14:06    [W:0.134 / U:0.184 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site