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. > > >
