On Thu, Feb 01, 2024, Mingwei Zhang wrote: > On Thu, Feb 01, 2024, Sean Christopherson wrote: > > On Wed, Jan 31, 2024, Mingwei Zhang wrote: > > > > The PMC is still active while the VM side handle_pmi_common() is not going to handle it? > > > > > > hmm, so the new value is '0', but the old value is non-zero, KVM is > > > supposed to zero out (stop) the fix counter), but it skips it. This > > > leads to the counter continuously increasing until it overflows, but > > > guest PMU thought it had disabled it. That's why you got this warning? > > > > No, that can't happen, and KVM would have a massive bug if that were the case. > > The truncation can _only_ cause bits to disappear, it can't magically make bits > > appear, i.e. the _only_ way this can cause a problem is for KVM to incorrectly > > think a PMC is being disabled. > > The reason why the bug does not happen is because there is global > control. So disabling a counter will be effectively done in the global > disable part, ie., when guest PMU writes to MSR 0x38f. > > fixed PMC is disabled. KVM will pause the counter in reprogram_counter(), and > > then leave the perf event paused counter as pmc_event_is_allowed() will return > > %false due to the PMC being locally disabled. > > > > But in this case, _if_ the counter is actually enabled, KVM will simply reprogram > > the PMC. Reprogramming is unnecessary and wasteful, but it's not broken. > > no, if the counter is actually enabled, but then it is assigned to > old_fixed_ctr_ctrl, the value is truncated. When control goes to the > check at the time of disabling the counter, KVM thinks it is disabled, > since the value is already truncated to 0. So KVM will skip by saying > "oh, the counter is already disabled, why reprogram? No need!". Ooh, I had them backwards. KVM can miss 1=>0, but not 0=>1. I'll apply this for 6.8; does this changelog work for you? Use a u64 instead of a u8 when taking a snapshot of pmu->fixed_ctr_ctrl when reprogramming fixed counters, as truncating the value results in KVM thinking all fixed counters, except counter 0, are already disabled. As a result, if the guest disables a fixed counter, KVM will get a false negative and fail to reprogram/disable emulation of the counter, which can leads to spurious PMIs in the guest.
