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

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Message-ID: <20220609113225.GA2326@willie-the-truck>
Date:   Thu, 9 Jun 2022 12:32:28 +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

[Please don't top-post]

On Thu, Jun 09, 2022 at 04:05:20PM +0530, Srinivasarao Pathipati wrote:
> On 6/9/2022 3:32 PM, Will Deacon wrote:
> > 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.
>
> On our Qualcomm platforms, The X bit is getting set by firmware at early
> bootup for Qualcomm use cases
> and non-secure world is resetting it, that causing issue.

What "Qualcomm use cases" and why should we care about them upstream?

Will