Re: [RFC PATCH 01/41] perf: x86/intel: Support PERF_PMU_CAP_VPMU_PASSTHROUGH

[Liang, Kan]

On 2024-04-11 4:43 p.m., Sean Christopherson wrote:
>>> And peeking ahead, IIUC perf effectively _forces_ a passthrough model when
>>> has_vpmu_passthrough_cap() is true, which is wrong.  There needs to be a user/admin
>>> opt-in (or opt-out) to that behavior, at a kernel/perf level, not just at a KVM
>>> level.  Hmm, or is perf relying on KVM to do that right thing?  I.e. relying on
>>> KVM to do perf_guest_{enter,exit}() if and only if the PMU can support the
>>> passthrough model.
>>>
>> Yes, perf relies on KVM to tell if a guest is entering the passthrough mode.
>>
>>> If that's the case, most of the has_vpmu_passthrough_cap() checks are gratiutous
>>> and confusing, e.g. just WARN if KVM (or some other module) tries to trigger a
>>> PMU context switch when it's not supported by perf.
>> If there is only non supported PMUs running in the host, perf wouldn't
>> do any context switch. The guest can feel free to use the core PMU. We
>> should not WARN for this case.
> I'm struggling to wrap my head around this.  If there is no supported PMU in the
> host, how can there be a core PMU for the guest to use?  KVM virtualizes a PMU
> if and only if kvm_init_pmu_capability() reports a compatible PMU, and IIUC that
> reporting is done based on the core PMU.
> 
> Specifically, I want to ensure we don't screw is passing through PMU MSR access,
> e.g. because KVM thinks perf will context switch those MSRs, but perf doesn't

Perf only context switches the MSRs of the PMU with the
PERF_PMU_CAP_VPMU_PASSTHROUGH flag. (Only the core PMU for this RFC).

For other PMUs without the PERF_PMU_CAP_VPMU_PASSTHROUGH, perf does
nothing in perf_guest_enter/exit().

KVM can rely on the flag to decide whether to enable the passthrough
mode for the PMU.

Thanks,
Kan