On Wed, Feb 2, 2022 at 6:43 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > Urgh... hate on kvm being a module again. We really need something like > EXPORT_SYMBOL_KVM() or something. Perhaps we should reconsider the current approach of treating the guest as a client of the host perf subsystem via kvm as a proxy. There are several drawbacks to the current approach: 1) If the guest actually sets the counter mask (and invert counter mask) or edge detect in the event selector, we ignore it, because we have no way of requesting that from perf. 2) If a system-wide pinned counter preempts one of kvm's thread-pinned counters, we have no way of letting the guest know, because the architectural specification doesn't allow counters to be suspended. 3) TDX is going to pull the rug out from under us anyway. When the TDX module usurps control of the PMU, any active host counters are going to stop counting. We are going to need a way of telling the host perf subsystem what's happening, or other host perf clients are going to get bogus data. Given what's coming with TDX, I wonder if we should just bite the bullet and cede the PMU to the guest while it's running, even for non-TDX guests. That would solve (1) and (2) as well.
