Re: KVM PMU virtualization

[Ingo Molnar <mingo@elte.hu>]

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

> 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.

You can still get those. You can even enable RDPMC access and avoid VM exits.

What you _cannot_ do is to 'steal' the PMU and just give it to the guest.

> 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.

Firstly, an emulated PMU was only the second-tier option i suggested. By far 
the best approach is native API to the host regarding performance events and 
good guest side integration.

Secondly, the PMU cannot be 'given' to the guest in the general case. Those 
are privileged registers. They can expose sensitive host execution details, 
etc. etc. So if you emulate a PMU you have to exit out of most PMU accesses 
anyway for a secure solution. (RDPMC can still be supported, but in close 
cooperation with the host)

> 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. [...]

You get my sure-fire NAK for that kind of crap though. Interfering with the 
host PMU and stealing it, is not a technical approach that has acceptable 
quality.

You need to integrate it properly so that host PMU functionality still works 
fine. (Within hardware constraints)

	Ingo