Re: [PATCH v2 00/11] KVM in-guest performance monitoring

[Avi Kivity <avi@redhat.com>]

On 06/15/2011 07:08 PM, David Ahern wrote:
> >  What does
> >  dmesg say about Perf?
>
> [    0.050995] Performance Events: Nehalem events, core PMU driver.
> [    0.051466] ... version:                1
> [    0.052998] ... bit width:              40
> [    0.053999] ... generic registers:      2
> [    0.054998] ... value mask:             000000ffffffffff
> [    0.055998] ... max period:             000000007fffffff
> [    0.057997] ... fixed-purpose events:   0
> [    0.058998] ... event mask:             0000000000000003

Well, it's not a Nehalem.  Can you tweak the model/family (via -cpu 
host) so it doesn't match a Nehalem and instead falls on the 
architectural PMU?

Trial-and-error should work to find a good combo.

> >
> >>  Also, the numbers for branches and branch-misses just seem wrong
> >>  compared to the same command run in the host as well as running
> >>  perf-stat in the host on the vcpu thread running openssl (with the vcpu
> >>  pinned to a pcpu).
> >
> >  Could be due to the fact that the counter is running in host mode.  Will
>
> You mean when perf is run in the guest?

Yes - it's counting host events (mostly kvm.ko) as well as guest events.

> >  be fixed once the exclude_host/exclude_guest patch makes it in (and
> >  gains Intel support).
> >
>
> How does exclude_{host,_guest} help if the guest-side counters are low
> -- by orders of magnitude?

It's probably the misidentification as Nehalem.

-- 
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