Re: Seeking a KVM benchmark

[Gleb Natapov <gleb@kernel.org>]

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.