Re: [PATCH v2 4/6] KVM: x86/pmu: Add pmc->intr to refactor kvm_perf_overflow{_intr}()

[Jim Mattson]

On Fri, Dec 10, 2021 at 2:59 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 12/10/21 23:55, Jim Mattson wrote:
> >>
> >> Even for tracing the SDM says "Like the value returned by RDTSC, TSC
> >> packets will include these adjustments, but other timing packets (such
> >> as MTC, CYC, and CBR) are not impacted".  Considering that "stand-alone
> >> TSC packets are typically generated only when generation of other timing
> >> packets (MTCs and CYCs) has ceased for a period of time", I'm not even
> >> sure it's a good thing that the values in TSC packets are scaled and offset.
> >>
> >> Back to the PMU, for non-architectural counters it's not really possible
> >> to know if they count in cycles or not.  So it may not be a good idea to
> >> special case the architectural counters.
> >
> > In that case, what we're doing with the guest PMU is not
> > virtualization. I don't know what it is, but it's not virtualization.
>
> It is virtualization even if it is incompatible with live migration to a
> different SKU (where, as you point out below, multiple TSC frequencies
> might also count as multiple SKUs).  But yeah, it's virtualization with
> more caveats than usual.

It's not virtualization if the counters don't count at the rate the
guest expects them to count.

> > Exposing non-architectural events is questionable with live migration,
> > and TSC scaling is unnecessary without live migration. I suppose you
> > could have a migration pool with different SKUs of the same generation
> > with 'seemingly compatible' PMU events but different TSC frequencies,
> > in which case it might be reasonable to expose non-architectural
> > events, but I would argue that any of those 'seemingly compatible'
> > events are actually not compatible if they count in cycles.
> I agree.  Support for marshaling/unmarshaling PMU state exists but it's
> more useful for intra-host updates than for actual live migration, since
> these days most live migration will use TSC scaling on the destination.
>
> Paolo
>
> >
> > Unless, of course, Like is right, and the PMU counters do count fractionally.
> >
>