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

[Robin Murphy]

On 2022-06-09 11:35, Srinivasarao Pathipati wrote:
> 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.

I think you're going to have to clarify what exactly this "issue" is if 
we're ever going to make sense of it...

I can't imagine that export from a disabled PMU would matter much, so my 
best guess is that EL2 firmware has reserved some counters via 
MDCR_EL2.HPMN which it's using to monitor the Non-Secure boot; if that 
also depends on PMCR.X remaining set, then as far as I can see it's 
really the firmware's own stupid fault for not using MDCR_EL2.TPMCR to 
prevent Linux from messing with its configuration. Or maybe something in 
the Secure world is trying to use the PMU independently and it's an EL3 
bug where PMCR_EL0 isn't being context-switched properly?

Robin.

> 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@xxxxxxxxxxx>
> > > ---
> > > 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
>
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