Re: Seeking a KVM benchmark

Gleb Natapov
2014-11-12T15:22:09Z

On Wed, Nov 12, 2014 at 12:33:32PM +0100, Paolo Bonzini wrote:
>
>
> On 10/11/2014 18:38, Gleb Natapov wrote:
> > On Mon, Nov 10, 2014 at 06:28:25PM +0100, Paolo Bonzini wrote:
> >> On 10/11/2014 15:23, Avi Kivity wrote:
> >>> It's not surprising [1]. Since the meaning of some PTE bits change [2],
> >>> the TLB has to be flushed. In VMX we have VPIDs, so we only need to flush
> >>> if EFER changed between two invocations of the same VPID, which isn't the
> >>> case.
> >>>
> >>> [1] after the fact
> >>> [2] although those bits were reserved with NXE=0, so they shouldn't have
> >>> any TLB footprint
> >>
> >> You're right that this is not that surprising after the fact, and that
> >> both Sandy Bridge and Ivy Bridge have VPIDs (even the non-Xeon ones).
> >> This is also why I'm curious about the Nehalem.
> >>
> >> However note that even toggling the SCE bit is flushing the TLB. The
> >> NXE bit is not being toggled here! That's the more surprising part.
> >>
> > Just a guess, but may be because writing EFER is not something that happens
> > often in regular OSes it is not optimized to handle different bits differently.
>
> Yes, that's what Intel said too.
>
> Nehalem results:
>
> userspace exit, urn 17560 17726 17628 17572 17417
> lightweight exit, urn 3316 3342 3342 3319 3328
> userspace exit, LOAD_EFER, guest!=host 12200 11772 12130 12164 12327
> lightweight exit, LOAD_EFER, guest!=host 3214 3220 3238 3218 3337
> userspace exit, LOAD_EFER, guest=host 11983 11780 11920 11919 12040
> lightweight exit, LOAD_EFER, guest=host 3178 3193 3193 3187 3220
>
Is this with Andy's patch that skips LOAD_EFER when guest=host, or the one
that always switch LOAD_EFER?

--
Gleb.