Re: [PATCH] perf/x86/amd: Reject AMD Merge event as a countable event

[Sandipan Das <sandipan.das@amd.com>]

On 16-09-2026 19:32, Absandze, Luka wrote:
> On 2026-09-16 12:51, sashiko-bot@kernel.org wrote:
>> [Severity: High]
>> Does returning a permanent -EINVAL error here cause an opportunistic retry storm
>> in KVM?
>>
>> If a KVM guest programs a Large Increment per Cycle (LIPC) event by writing
>> AMD_MERGE_EVENT to an odd vPMC, KVM intercepts this and attempts to create a
>> raw host perf event.  When this fails with -EINVAL, KVM's reprogram_counter()
>> propagates the error to kvm_pmu_handle_event():
>>
>> arch/x86/kvm/pmu.c:kvm_pmu_handle_event() {
>>     ...
>>                 if (reprogram_counter(pmc))
>>                         set_bit(pmc->idx, pmu->reprogram_pmi);
>>     ...
>> }
>>
>> Because KVM treats all failures as transient and unconditionally re-adds the
>> failing counter to pmu->reprogram_pmi, on every subsequent PMU refresh KVM
>> will repeatedly retry the failed counter.
>>
>> Could this create a permanent retry storm that degrades VM performance and
>> wastes host CPU cycles?
>>
> 
> There ins't any substantial load generated, we short circuit here before
> the expesnive parts. However, a followup in KVM could be beneficial to
> reject this case outright.
> 
> I'll wait for opinions

I think it makes sense for the host layer to reject PMCxFFF altogether. Today,
at least for emulated vPMU, when a KVM guest programs a LIPC event, 3 PMCs are
consumed. This will ensure that the 3rd one, initiated by the guest programming
PMCxFFF, is dropped.

I also agree that a followup in KVM is required.