Re: KVM PMU virtualization

Avi Kivity
2010-02-26T12:23:50Z

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.

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.

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).

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.