Re: KVM PMU virtualization

[Ingo Molnar <mingo@elte.hu>]

* Jes Sorensen <Jes.Sorensen@redhat.com> wrote:

> > Agree about favouring modern processors.
> 
> You certainly cannot emulate the Core2 on a P4. The Core2 is Perfmon v2, 
> whereas Nehalem and Atom are v3 if I remember correctly. [...]

Of course you can emulate a good portion of it, as long as there's perf 
support on the host side for P4.

If the guest programs a cachemiss event, you program a cachemiss perf event on 
the host and feed its values to the emulated MSR state. You _dont_ program the 
raw PMU on the host side - just use the API i outlined to get struct 
perf_event.

The emulation wont be perfect: not all events will count and not all events 
will be available in a P4 (and some Core2 events might not even make sense in 
a P4), but that is reality as well: often documented events dont count, and 
often non-documented events count.

What matters to 99.9% of people who actually use this stuff is a few core sets 
of events - which are available in P4s and in Core2 as well. Cycles, 
instructions, branches, maybe cache-misses. Sometimes FPU stuff.

For Linux<->Linux the sanest, tier-1 approach would be to map sys_perf_open() 
on the guest side over to the host, transparently, via a paravirt driver.

	Ingo