Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU state for Intel CPU

[Mingwei Zhang]

> >
> > 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