On 2024-04-25 5:46 p.m., Sean Christopherson wrote: > On Thu, Apr 25, 2024, Kan Liang wrote: >> On 2024-04-25 4:16 p.m., Mingwei Zhang wrote: >>> On Thu, Apr 25, 2024 at 9:13 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote: >>>> It should not happen. For the current implementation, perf rejects all >>>> the !exclude_guest system-wide event creation if a guest with the vPMU >>>> is running. >>>> However, it's possible to create an exclude_guest system-wide event at >>>> any time. KVM cannot use the information from the VM-entry to decide if >>>> there will be active perf events in the VM-exit. >>> >>> Hmm, why not? If there is any exclude_guest system-wide event, >>> perf_guest_enter() can return something to tell KVM "hey, some active >>> host events are swapped out. they are originally in counter #2 and >>> #3". If so, at the time when perf_guest_enter() returns, KVM will ack >>> that and keep it in its pmu data structure. >> >> I think it's possible that someone creates !exclude_guest event after > > I assume you mean an exclude_guest=1 event? Because perf should be in a state > where it rejects exclude_guest=0 events. > Right. >> the perf_guest_enter(). The stale information is saved in the KVM. Perf >> will schedule the event in the next perf_guest_exit(). KVM will not know it. > > Ya, the creation of an event on a CPU that currently has guest PMU state loaded > is what I had in mind when I suggested a callback in my sketch: > > : D. Add a perf callback that is invoked from IRQ context when perf wants to > : configure a new PMU-based events, *before* actually programming the MSRs, > : and have KVM's callback put the guest PMU state > > It's a similar idea to TIF_NEED_FPU_LOAD, just that instead of a common chunk of > kernel code swapping out the guest state (kernel_fpu_begin()), it's a callback > into KVM. Yes, a callback should be required. I think it should be done right before switching back to the host perf events, so there are an accurate active event list. Thanks, Kan
