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When this fails with -EINVAL, KVM's reprogram_counter() >> propagates the error to kvm_pmu_handle_event(): >> >> arch/x86/kvm/pmu.c:kvm_pmu_handle_event() { >> ... >> if (reprogram_counter(pmc)) >> set_bit(pmc->idx, pmu->reprogram_pmi); >> ... >> } >> >> Because KVM treats all failures as transient and unconditionally re-adds the >> failing counter to pmu->reprogram_pmi, on every subsequent PMU refresh KVM >> will repeatedly retry the failed counter. >> >> Could this create a permanent retry storm that degrades VM performance and >> wastes host CPU cycles? >> > > There ins't any substantial load generated, we short circuit here before > the expesnive parts. However, a followup in KVM could be beneficial to > reject this case outright. > > I'll wait for opinions I think it makes sense for the host layer to reject PMCxFFF altogether. Today, at least for emulated vPMU, when a KVM guest programs a LIPC event, 3 PMCs are consumed. This will ensure that the 3rd one, initiated by the guest programming PMCxFFF, is dropped. I also agree that a followup in KVM is required.