Re: [kvm-unit-tests patch v6 13/18] x86: pmu: Improve instruction and branches events verification

Mi, Dapeng
2025-02-18T09:40:33Z

On 2/15/2025 5:08 AM, Sean Christopherson wrote:
> On Sat, Sep 14, 2024, Dapeng Mi wrote:
>> If HW supports GLOBAL_CTRL MSR, enabling and disabling PMCs are moved in
>> __precise_count_loop(). Thus, instructions and branches events can be
>> verified against a precise count instead of a rough range.
>>
>> BTW, some intermittent failures on AMD processors using PerfMonV2 is
>> seen due to variance in counts. This probably has to do with the way
>> instructions leading to a VM-Entry or VM-Exit are accounted when
>> counting retired instructions and branches.
> AMD counts VMRUN as a branch in guest context.

Good to know. Thanks.

>
>> + * We see some intermittent failures on AMD processors using PerfMonV2
>> + * due to variance in counts. This probably has to do with the way
>> + * instructions leading to a VM-Entry or VM-Exit are accounted when
>> + * counting retired instructions and branches. Thus only enable the
>> + * precise validation for Intel processors.
>> + */
>> + if (pmu.is_intel && this_cpu_has_perf_global_ctrl()) {
>> + /* instructions event */
> These comments are useless.

Sure.

>
>> + gp_events[instruction_idx].min = LOOP_INSTRNS;
>> + gp_events[instruction_idx].max = LOOP_INSTRNS;
>> + /* branches event */
>> + gp_events[branch_idx].min = LOOP_BRANCHES;
>> + gp_events[branch_idx].max = LOOP_BRANCHES;
>> + }
>> +}