> > > > Maybe, (just maybe), it is possible to do PMU context switch at vcpu > > boundary normally, but doing it at VM Enter/Exit boundary when host is > > profiling KVM kernel module. So, dynamically adjusting PMU context > > switch location could be an option. > If there are two VMs with pmu enabled both, however host PMU is not > enabled. PMU context switch should be done in vcpu thread sched-out path. > > If host pmu is used also, we can choose whether PMU switch should be > done in vm exit path or vcpu thread sched-out path. > host PMU is always enabled, ie., Linux currently does not support KVM PMU running standalone. I guess what you mean is there are no active perf_events on the host side. Allowing a PMU context switch drifting from vm-enter/exit boundary to vcpu loop boundary by checking host side events might be a good option. We can keep the discussion, but I won't propose that in v2. I guess we are off topic. Sean's suggestion is that we should put "perf" and "kvm" together while doing the context switch. I think this is quite reasonable regardless of the PMU context switch location. To execute this, I am thinking about adding a parameter or return value to perf_guest_enter() so that once it returns back to KVM, KVM gets to know which counters are active/inactive/cleared from the host side. Knowing that, KVM can do the context switch more efficiently. Thanks. -Mingwei
