Re: [PATCH v2 0/4] perf/x86: Don't write PEBS_ENABLED on KVM transitions

[Mi, Dapeng]

On 4/24/2026 12:16 AM, Peter Zijlstra wrote:
> On Thu, Apr 23, 2026 at 08:03:36AM -0700, Sean Christopherson wrote:
>> Testing this against our "PEBS_ENABLED is stuck" reproducer is (still) a work
>> in-progress (largely because the "reproducer" is currently "throw the kernel in
>> a big test pool"), i.e. I don't know if this actually resolves the problems we
>> are seeing.  But even if it doesn't fully resolve our woes, it seems like a
>> no-brainer improvement, and if we're missing something with respect to "stuck"
>> PEBS_ENABLED, it'd be nice to get feedback/input asap.
>>
>> Note, if the throttling theory is correct (which is looking unlikely at the
>> moment), then there are likely more fixes that need to be done, e.g. for CPUs
>> without isolation, and/or if PERF_GLOBAL_CTRL can be modified from NMI context
>> too.
> Throttle does: pmu->stop() := x86_pmu_stop() -> intel_pmu_disable_event()
>
> Which in turn should:
>
>   x86_pmu_disable_event()
>     wrmsrq(config_base, config & ~EN);
>   x86_pmu_pebs_disable() := intel_pmu_pebs_disable()
>     wrmsr(PEBS_ENABLE, pebs_enabled & ~(1<<idx));
>
> So that's just the counter EN bit and PEBS_ENABLED cleared. However, if
> this is from PMI, then the PMI handler should also update GLOBAL_CTRL --
> provided it wasn't 0.
>
> See intel_pmu_handle_irq():
>
>   if (pmu_enabled)
>   	__intel_pmu_enable_all()
> 	  wrmsrq(GLOBAL_CTRL, intel_ctrl);
>
Yes, currently all valid bits in GLOBAL_CTRL would be set by default on
Intel platforms. IIUC, this issue looks more like a race condition between
Perf and KVM.

1. KVM saves the value of host PEBS_ENABLE before VM-entry.

2. PMI is triggered and interrupts the upcoming VM-entry. PEBS events are
throttled and PEBS_ENABLE MSR is updated in the PMI handler, then the KVM
saved host PEBS_ENABLE value gets stale. 

3. VM entry continues and then the next VM-exit occurs, the stale
PEBS_ENABLE value is restored. 

4. The PEBS_ENABLE MSR keeps the stale value until next write.

Seems an alternative way to fix this issue is to disable the PMU (Clearing
GLOBAL_CTRL) before KVM saving the PMU MSRs?

Thanks.