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. 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. 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. So I prefer 3. Thanks, Kan >> But I prefer a more generic and straightforward function name, e.g., >> intel_pt_stop_save()/intel_pt_start_load(), in case other modules may want to >> save/restore the PT information in their context switch later. >> >> Thanks, >> Kan >> >>> >>>> It seems perf_event_disable() is not used widely by other kernel >>>> component. > > Because there are not lots of kernel users. > You can check another user, watchdog_hld.c, perf_event_enable/disable are used there.
