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

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On Thu, Apr 25, 2024, Kan Liang wrote:
> On 2024-04-25 4:16 p.m., Mingwei Zhang wrote:
> > On Thu, Apr 25, 2024 at 9:13 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
> >> It should not happen. For the current implementation, perf rejects all
> >> the !exclude_guest system-wide event creation if a guest with the vPMU
> >> is running.
> >> However, it's possible to create an exclude_guest system-wide event at
> >> any time. KVM cannot use the information from the VM-entry to decide if
> >> there will be active perf events in the VM-exit.
> > 
> > Hmm, why not? If there is any exclude_guest system-wide event,
> > perf_guest_enter() can return something to tell KVM "hey, some active
> > host events are swapped out. they are originally in counter #2 and
> > #3". If so, at the time when perf_guest_enter() returns, KVM will ack
> > that and keep it in its pmu data structure.
> I think it's possible that someone creates !exclude_guest event after

I assume you mean an exclude_guest=1 event?  Because perf should be in a state
where it rejects exclude_guest=0 events.

> the perf_guest_enter(). The stale information is saved in the KVM. Perf
> will schedule the event in the next perf_guest_exit(). KVM will not know it.

Ya, the creation of an event on a CPU that currently has guest PMU state loaded
is what I had in mind when I suggested a callback in my sketch:

 :  D. Add a perf callback that is invoked from IRQ context when perf wants to
 :     configure a new PMU-based events, *before* actually programming the MSRs,
 :     and have KVM's callback put the guest PMU state

It's a similar idea to TIF_NEED_FPU_LOAD, just that instead of a common chunk of
kernel code swapping out the guest state (kernel_fpu_begin()), it's a callback
into KVM.

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