Re: [PATCH RFC v2 00/12] xen/x86: use per-vcpu stacks for 64 bit pv domains

[Dario Faggioli <dfaggioli@suse.com>]

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Hey, Hi!

On Mon, 2018-01-22 at 18:39 +0000, Andrew Cooper wrote:
> > > > On 22.01.18 at 15:38, <jgross@suse.com> wrote:
> > > I'm quite sure the performance will be much better as it doesn't
> > > require
> > > per physical cpu L4 page tables, but just a shadow L4 table for
> > > each
> > > guest L4 table, similar to the Linux kernel KPTI approach.
> > 
> Juergen: you're now adding a LTR into the context switch path which
> tends to be very slow.  I.e. As currently presented, this series
> necessarily has a higher runtime overhead than Jan's XPTI.
> 
So, as Juergen mentioned, I'm trying to do some performance evaluation
of these solutions.

This is just the first set of numbers, so consider it preliminary. In
particular, I'm sure there is a better set of benchmarks than the ones
I've used for now (in order to have something quickly)... I am looking
more into this.

Anyway, what I'm seeing for now is that Juergen's branch performs
pretty much as current staging, if booted with xpti=false (i.e., with
Jan's band-aid compiled but disabled).

OTOH, staging with xpti=true does show some performance impact. I
appreciate that this is still unfair a comparison (as Juergen's series
lacks the "real XPTI" bits), but the goal here was to figure out
whether the current status of the series is already introducing
regressions or not (and, as far as this first set of benches says, it's
not).

Anyway, here's the numbers. The benchmarks are run in a 16 vCPUs Debian
PV guest, on a 16 pCPUs (Intel Xeon-s) Debian host.

Raw numbers:
https://openbenchmarking.org/result/1801238-AL-1801232AL05

Normalized against "Staging xpti=false"
https://openbenchmarking.org/result/1801238-AL-1801232AL05&obr_nor=y&obr_hgv=4.11+Staging+xpti%3Dfalse

You'll have to forgive me about the labels (I'll pick better titles
next time). They're meaning is as follows:
- - "4.11 Staging xpti=false" this is current staging, booted with
  xpti=false (so, with Jan's band-aid applied, but disabled);
- - "staging-xpti-on" this is current staging, booted with xpti=true
  (so, with Jan's band-aid applied, but enabled);
- - "4.11 Juergen xpti" this is Juergen's GitHub branch, booted with
xpti=true.

I'll post more as soon as I will have it.
Dario
- -- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
- -----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Software Engineer @ SUSE https://www.suse.com/
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=/v47
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