linux-kernel - Re: [kvm-unit-tests patch v6 17/18] x86: pmu: Adjust lower boundary of branch-misses event

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Message-ID: <Z6-w_ZG1LmTYDJd1@google.com>
Date: Fri, 14 Feb 2025 13:09:17 -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 17/18] x86: pmu: Adjust lower boundary
 of branch-misses event

On Sat, Sep 14, 2024, Dapeng Mi wrote:
> @@ -205,6 +208,17 @@ static void adjust_events_range(struct pmu_event *gp_events,
>  		gp_events[branch_idx].min = LOOP_BRANCHES;
>  		gp_events[branch_idx].max = LOOP_BRANCHES;
>  	}
> +
> +	/*
> +	 * For CPUs without IBPB support, no way to force to trigger a
> +	 * branch miss and the measured branch misses is possible to be
> +	 * 0. Thus overwrite the lower boundary of branch misses event
> +	 * to 0 to avoid false positive.
> +	 */
> +	if (!has_ibpb()) {
> +		/* branch misses event */

This comment is worse than useless, because it necessitates curly braces.

> +		gp_events[branch_miss_idx].min = 0;
> +	}
>  }