linux-kernel - Re: [PATCH V5] arm64: perf: Make exporting of pmu events configurable

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Message-ID: <20220609100247.GA2187@willie-the-truck>
Date:   Thu, 9 Jun 2022 11:02:47 +0100
From:   Will Deacon <will@...nel.org>
To:     Srinivasarao Pathipati <quic_c_spathi@...cinc.com>
Cc:     mark.rutland@....com, peterz@...radead.org, mingo@...hat.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...nel.org, namhyung@...nel.org, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5] arm64: perf: Make exporting of pmu events configurable

On Mon, May 23, 2022 at 05:19:34PM +0530, Srinivasarao Pathipati wrote:
> The PMU export bit (PMCR_EL0.X) is getting reset during pmu reset,
> Make is configurable using sysctls to enable/disable at runtime.
> It can also be enabled at early bootup with kernel arguments.
> 
> Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@...cinc.com>
> ---
> Changes since V4:
> 	- Registering sysctls dynamically for only arm64 as suggested by Will
> 	- Not removed the code to configure with kernel parameters 
> 	  as the sysctl's kernel parameter(sysctl.kernel.export_pmu_events)
> 	  is not working at early bootup. pmu_reset() getting called before 
> 	  sysctl's kernel parameter is set.

Why do you need this during early bootup? Perf won't program any events
until much later and if somebody else is configuring the PMU before
entering Linux then they can also set that X bit in the PMCR.

Will