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Peter Anvin" , Andy Lutomirski , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Paolo Bonzini , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, loongarch@lists.linux.dev, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Mingwei Zhang , Xudong Hao , Sandipan Das , Dapeng Mi , Xiong Zhang , Manali Shukla , Jim Mattson Content-Type: text/plain; charset="us-ascii" On Mon, Dec 08, 2025, Peter Zijlstra wrote: > On Fri, Dec 05, 2025 at 04:16:40PM -0800, Sean Christopherson wrote: > > > +static atomic_t nr_include_guest_events __read_mostly; > > + > > +static atomic_t nr_mediated_pmu_vms __read_mostly; > > +static DEFINE_MUTEX(perf_mediated_pmu_mutex); > > > +static int mediated_pmu_account_event(struct perf_event *event) > > +{ > > + if (!is_include_guest_event(event)) > > + return 0; > > + > > + guard(mutex)(&perf_mediated_pmu_mutex); > > + > > + if (atomic_read(&nr_mediated_pmu_vms)) > > + return -EOPNOTSUPP; > > + > > + atomic_inc(&nr_include_guest_events); > > + return 0; > > +} > > + > > +static void mediated_pmu_unaccount_event(struct perf_event *event) > > +{ > > + if (!is_include_guest_event(event)) > > + return; > > + > > + atomic_dec(&nr_include_guest_events); > > +} > > > +int perf_create_mediated_pmu(void) > > +{ > > + guard(mutex)(&perf_mediated_pmu_mutex); > > + if (atomic_inc_not_zero(&nr_mediated_pmu_vms)) > > + return 0; > > + > > + if (atomic_read(&nr_include_guest_events)) > > + return -EBUSY; > > + > > + atomic_inc(&nr_mediated_pmu_vms); > > + return 0; > > +} > > +EXPORT_SYMBOL_GPL(perf_create_mediated_pmu); > > + > > +void perf_release_mediated_pmu(void) > > +{ > > + if (WARN_ON_ONCE(!atomic_read(&nr_mediated_pmu_vms))) > > + return; > > + > > + atomic_dec(&nr_mediated_pmu_vms); > > +} > > +EXPORT_SYMBOL_GPL(perf_release_mediated_pmu); > > These two things are supposed to be symmetric, but are implemented > differently; what gives? > > That is, should not both have the general shape: > > if (atomic_inc_not_zero(&A)) > return 0; > > guard(mutex)(&lock); > > if (atomic_read(&B)) > return -EBUSY; > > atomic_inc(&A); > return 0; > > Similarly, I would imagine both release variants to have the underflow > warn on like: > > if (WARN_ON_ONCE(!atomic_read(&A))) > return; > > atomic_dec(&A); > > Hmm? IIUC, you're suggesting someting like this? If so, that makes perfect sense to me. diff --git a/kernel/events/core.c b/kernel/events/core.c index c6368c64b866..fa2e7b722283 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -6356,7 +6356,8 @@ static int mediated_pmu_account_event(struct perf_event *event) static void mediated_pmu_unaccount_event(struct perf_event *event) { - if (!is_include_guest_event(event)) + if (!is_include_guest_event(event) || + WARN_ON_ONCE(!atomic_read(&nr_include_guest_events))) return; atomic_dec(&nr_include_guest_events); > Also, EXPORT_SYMBOL_FOR_KVM() ? Ya, for sure. I posted this against a branch without EXPORT_SYMBOL_FOR_KVM(), because there are also hard dependencies on the for-6.19 KVM pull requests, and I didn't want to wait to post until 6.19-rc1 because of the impending winter break. Though I also simply forgot about these exports :-( These could also use EXPORT_SYMBOL_FOR_KVM(): EXPORT_SYMBOL_FOR_MODULES(perf_load_guest_lvtpc, "kvm"); EXPORT_SYMBOL_FOR_MODULES(perf_put_guest_lvtpc, "kvm"); > I can make these edits when applying, if/when we get to applying. Let me > continue reading. >