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

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Message-ID: <Z6-w24T1iH2S_Fux@google.com>
Date: Fri, 14 Feb 2025 13:08:43 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.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>, Zhenyu Wang <zhenyuw@...ux.intel.com>, 
	Like Xu <like.xu.linux@...il.com>, Jinrong Liang <cloudliang@...cent.com>, 
	Yongwei Ma <yongwei.ma@...el.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 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.

> +	 * 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.

> +		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;
> +	}
> +}