On Thu, Mar 20, 2025 at 09:19:02AM +0000, Marc Zyngier wrote: > On Wed, 19 Mar 2025 18:51:28 +0000, Oliver Upton <[email protected]> > wrote: > > I'm at least willing to plug my nose and do the following: > > > > 1) When the VMM does not specify a vPMU type: > > > > - We continue to present the 'default' PMU (including event counters) > > to the VM > > > > - KVM ensures that the fixed CPU cycle counter works on any PMUv3 > > implementation in the system, even if it is different from the > > default > > > > - Otherwise, event counters will only count on the default > > implementation and will not count on different PMUs > > I think this is confusing. The CC is counting, but nothing else, and > people using the cycle counters in conjunction with other events (a > very common use case) will not be able to correlate things correctly. > The current behaviour is, for all its sins, at least consistent.
You of course have a good point. What Windows is doing is definitely an outlier. > > > > 2) Implement your suggestion of a UAPI where the VMM can select a PMU > > that only has the CPU cycle counter and works on any PMUv3 > > implementation. > > > > Either way KVM will need to have some special case handling of the fixed > > CPU cycle counter. That'd allow users to actually run Windows *now* and > > provide a clear mechanism for userspace to present a less-broken vPMU if > > it cares. > > Honestly, I don't care about one guest or another. My point is that if > we are changing the behaviour of the PMU to deal with this sort of > things, then it has to be a userspace buy-in. I'm fine with just the user buy-in then. But I still do care about the guest compatibility issue, especially since the end user of all this crap is unlikely to know/care about the fine details of the implementation. So, Akihiko, I would *greatly* appreciate it if you propose a complete solution to the problem, including the KVM and VMM patches to make it all work. Thanks, Oliver
