Re: KVM PMU virtualization

[Ingo Molnar <mingo@elte.hu>]

* Avi Kivity <avi@redhat.com> wrote:

> On 02/26/2010 02:07 PM, Ingo Molnar wrote:
> >* Avi Kivity<avi@redhat.com>  wrote:
> >
> >>A native API to the host will lock out 100% of the install base now, and a
> >>large section of any future install base.
> >... which is why i suggested the soft-PMU approach.
> 
> Not sure I understand it completely.
> 
> Do you mean to take the model specific host pmu events, and expose them to 
> the guest via trap'n'emulate?  In that case we may as well assign the host 
> pmu to the guest if the host isn't using it, and avoid the traps.

You are making the incorrect assumption that the emulated PMU uses up all host 
PMU resources ...

> Do you mean to choose some older pmu and emulate it using whatever pmu model 
> the host has?  I haven't checked, but aren't there mutually exclusive events 
> in every model pair?  The closest thing would be the architectural pmu 
> thing.

Yes, something like Core2 with 2 generic events.

That would leave 2 extra generic events on Nehalem and better. (which is 
really the target CPU type for any new feature we are talking about right now. 
Plus performance analysis tends to skew towards more modern CPU types as 
well.)

Plus the emulation can be smart about it and only use up a given number. Most 
guest OSs dont use the full PMU - they use a single counter.

Ideally for Linux<->Linux there would be a PMU paravirt driver that allocates 
events on an as-needed basis.

> Or do you mean to define a new, kvm-specific pmu model and feed it off the 
> host pmu?  In this case all the guests will need to be taught about it, 
> which raises the compatibility problem.
>
> > And note that _any_ solution we offer locks out 100% of the installed base 
> > right now, as no solution is in the kernel yet. The only question is what 
> > kind of upgrade effort is needed for users to make use of the feature.
> 
> I meant the guest installed base.  Hosts can be upgraded transparently to 
> the guests (not even a shutdown/reboot).

The irony: this time guest-transparent solutions that need no configuration 
are good? ;-)

The very same argument holds for the file server thing: a guest transparent 
solution is easier wrt. the upgrade path.

	Ingo