Re: KVM PMU virtualization

[Jes Sorensen <Jes.Sorensen@redhat.com>]

On 02/26/10 11:44, Ingo Molnar wrote:
> Direct access to counters is not something that is a big issue. [ Given that i
> sometimes can see KVM redraw the screen of a guest OS real-time i doubt this
> is the biggest of performance challenges right now ;-) ]
>
> By far the biggest instrumentation issue is:
>
>   - availability
>   - usability
>   - flexibility
>
> Exposing the raw hw is a step backwards in many regards. The same way we dont
> want to expose chipsets to the guest to allow them to do RAS. The same way we
> dont want to expose most raw PCI devices to guest in general, but have all
> these virt driver abstractions.

I have to say I disagree on that. When you run perfmon on a system, it
is normally to measure a specific application. You want to see accurate
numbers for cache misses, mul instructions or whatever else is selected.
Emulating the PMU rather than using the real one, makes the numbers far
less useful. The most useful way to provide PMU support in a guest is
to expose the real PMU and let the guest OS program it.

We can do this in a reasonable way today, if we allow to take the PMU
away from the host, and only let guests access it when it's in use.
Hopefully Intel and AMD will come up with proper hw PMU virtualization
support that allows us to do it 100% guest and host at some point.

Cheers,
Jes