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

[maobibo]

On 2024/4/24 上午1:02, Mingwei Zhang wrote:
> > > 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.
yeap, that sounds great.

Regards
Bibo Mao

> Thanks.
> -Mingwei