Re: [PATCH v2 1/4] perf/x86/intel: Don't write PEBS_ENABLED on host<=>guest xfers if CPU has isolation

[Mi, Dapeng]

On 4/28/2026 2:07 AM, Sean Christopherson wrote:
> On Mon, Apr 27, 2026, Dapeng Mi wrote:
>> On 4/24/2026 1:59 AM, Jim Mattson wrote:
>>>>  arch/x86/events/intel/core.c | 42 ++++++++++++++++++++----------------
>>>>  1 file changed, 23 insertions(+), 19 deletions(-)
>>>>
>>>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>>>> index 793335c3ce78..002d809f82ef 100644
>>>> --- a/arch/x86/events/intel/core.c
>>>> +++ b/arch/x86/events/intel/core.c
>>>> @@ -4999,12 +4999,15 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
>>>>         struct kvm_pmu *kvm_pmu = (struct kvm_pmu *)data;
>>>>         u64 intel_ctrl = hybrid(cpuc->pmu, intel_ctrl);
>>>>         u64 pebs_mask = cpuc->pebs_enabled & x86_pmu.pebs_capable;
>>>> -       int global_ctrl, pebs_enable;
>>>> +       u64 guest_pebs_mask = pebs_mask & ~cpuc->intel_ctrl_host_mask;
>>>> +       int global_ctrl;
>>> Is it worth noting somewhere that pebs_ept is not supported on any
>>> CPUs with PMU version < 5, where a single event can set two
>>> PEBS_ENABLE bits (cf. intel_pmu_pebs_enable)?
> Is that a hardware limitation, or a "perf hasn't added pebs_ept for PMU v5 yet"
> thing?  I assume it's the latter?
>
>>>> +       /*
>>>> +        * Disable counters where the guest PMC is different than the host PMC
>>>> +        * being used on behalf of the guest, as the PEBS record includes
>>>> +        * PERF_GLOBAL_STATUS, i.e. the guest will see overflow status for the
>>>> +        * wrong counter(s).  Similarly, disallow PEBS in the guest if the host
>>>> +        * is using PEBS, to avoid bleeding host state into PEBS records.
>>>> +        */
>>>> +       guest_pebs_mask &= kvm_pmu->pebs_enable & ~kvm_pmu->host_cross_mapped_mask;
>>>> +       if (pebs_mask & ~cpuc->intel_ctrl_guest_mask)
>>>> +               guest_pebs_mask = 0;
>>> I don't understand this clause. IIUC, it says that if we don't have
>>> any exclude-host PEBS events, then clear PEBS_ENABLE for the guest.
>> I suppose it says all guest PEBS events need to be disabled if there is any
>> event using PEBS on host side, and it's clearing GLOBAL_CTRL instead of
>> PEBS_ENABLE to disable guest PEBS events. 
> Yeah, but why disable _everything_?
>
>>> Yes, any guest-programmed PEBS event should be exclude-host, but if
>>> there is an inconsistency, shouldn't we apply a mask? What if there is
>>> only one exclude-host PEBS event, but there are two bits set in
>>> guest_pebs_mask?
> I'm confused about this as well.  The comment above about not bleeding host state
> into the PEBS records is my best guess (and it's probably not a very good guess)
> as to why the code does what it does.  The changelog from commit 854250329c02
> ("KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations") just says:
>     
>     The guest PEBS will be disabled when some users try to perf KVM and
>     its user-space through the same PEBS facility 
>
> That doesn't entirely make sense to me though, because I would think disabling
> the host counters iva GLOBAL_CTRL would suffice.  I.e. there's no need to disallow
> PEBS in the guest just because the host is also using PEBS.  But I can't think of
> any other reason to fully disable PEBS.
>
> FWIW, I was going off the previous code which effectively did:
>
> 	if (cpuc->pebs_enabled & ~cpuc->intel_ctrl_guest_mask) {
> 		/* Disable guest PEBS if host PEBS is enabled. */
> 		arr[pebs_enable].guest = 0;
> 	}
>
> One idea would be to add a FIXME, and then address the FIXME in a follow-up patch
> (in the same series)?  And then see what breaks? :-)

I'm confused with this. does this go back to manipulate PEBS_ENABLE again? 

I suppose what Jim mentioned is that the PEBS_LDLAT or PEBS_ST events could
set multiple bits in PEBS_ENABLE MSR before Perfmon v5 as below code shows.

```

void intel_pmu_pebs_enable(struct perf_event *event)
{

......

if ((event->hw.flags & PERF_X86_EVENT_PEBS_LDLAT) && (x86_pmu.version < 5))
        cpuc->pebs_enabled |= 1ULL << (hwc->idx + 32);
    else if (event->hw.flags & PERF_X86_EVENT_PEBS_ST)
        cpuc->pebs_enabled |= 1ULL << 63;

......

```

This would lead to multiple bits set in pebs_mask and may cause incorrect
results.

If so, maybe we can bit-and the pebs_mask with intel_ctrl to filter out
these extra bits?