On Thursday, September 15, 2022 9:55 PM Liang, Kan wrote: > On 2022-09-14 10:46 p.m., Wang, Wei W wrote: > > On Thursday, September 15, 2022 4:26 AM, Liang, Kan wrote: > >> The perf_event_disable() eventually invokes the intel_pt_stop(). > >> We already expose the intel_pt_stop()/cpu_emergency_stop_pt() to > >> other modules. I don't think we have to use the perf_event_disable(). > >> Also, the > >> perf_event_disable() requires extra codes. > >> > >> I went through the discussions. I agree with Sean's suggestion. > >> We should only put the logic in the KVM but all the MSR access > >> details into the PT driver. > > > > Even the driver itself doesn’t drive the save/restore of the MSRs, it is drived > by perf. > > It through perf_event, not driven by perf_event. The perf_event generic code > never knows when should invokes each driver to save/restore information. It > should be driven by the other subsystem e.g., scheduler. Yes. The cpu scheduler does this via the perf subsystem, though. > > For this case, KVM should drive the save/restore, and the PT driver eventually > does all the MSR access details. > > > 1. If we make KVM a user of perf, we should do this via > perf_event_disable/enable_*. > > 2. If we make KVM an alternative to perf (i.e. have direct control > > over PMU HW), we can do this via driver interfaces like perf. > > Per my experience, we should go for 1. Probably need Peter's opinions on > this. > > > > For 1, the perf_event_disable/enable_* are not enough. They don't > save/restore MSRs. perf_event_disable will go through perf to call pt_event_stop which saves the related MSRs, right? (if so, what large changes did you mean?) > If we go to this way, we have to introduce a new generic > interface to ask each driver to save/restore their MSRs when the guest is > entering/exiting. We'd better combine the new interface with the existing > perf_guest_get_msrs() of the core driver. > I think that's an ideal solution, but requires big changes in the code. > > 2 is the current KVM implementation. See pt_save_msr()/pt_load_msr(). I don't > think it's a right way. We'd better fix it. > > The suggestion should be 3. The KVM notify the PT driver via the interface > provided by PT. The PT driver save/restore all the registers. > I think it's an acceptable solution with small code changes. This looks like we just relocate the save/restore functions to the PT driver and KVM still directly call them - still not going through perf's management. Imagine every user operates on the pmu h/w directly like this, things would be a mess. Thanks, Wei
