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

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Message-ID: <5381f994-54db-458c-b32f-aa0dfb696d3a@linux.intel.com>
Date: Tue, 18 Feb 2025 17:40:21 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
 Mingwei Zhang <mizhang@...gle.com>, Xiong Zhang <xiong.y.zhang@...el.com>,
 Like Xu <like.xu.linux@...il.com>, Jinrong Liang <cloudliang@...cent.com>,
 Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [kvm-unit-tests patch v6 13/18] x86: pmu: Improve instruction and
 branches events verification


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;
>> +	}
>> +}