Re: [PATCH V6] arm64: perf: Make exporting of pmu events configurable

[Srinivasarao Pathipati]

On 6/24/2022 5:48 PM, Will Deacon wrote:
> On Thu, Jun 09, 2022 at 06:29:49PM +0530, Srinivasarao Pathipati wrote:
> > The PMU export bit (PMCR_EL0.X) is getting reset during pmu reset,
> > Make it configurable using sysctls to enable/disable at runtime.
> >
> > Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@xxxxxxxxxxx>
> > ---
> > Changes since V5:
> > 	- removed configuring with kernel parameters.
> > 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.
> > Changes since V3:
> > 	- export bit is now configurable with sysctl
> > 	- enabling export bit on reset instead of retaining
> >
> > Changes since V2:
> > 	Done below changes as per Will's comments
> > 	- enabling pmcr_x now configurable with kernel parameters and
> > 	  by default it is disabled.
> > 	
> > Changes since V1:
> > 	- Preserving only PMCR_X bit as per Robin Murphy's comment.
> >
> > ---
> >   Documentation/admin-guide/sysctl/kernel.rst | 11 +++++++++++
> >   arch/arm64/kernel/perf_event.c              | 13 +++++++++++++
> >   2 files changed, 24 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> > index ddccd10..c2ecd84 100644
> > --- a/Documentation/admin-guide/sysctl/kernel.rst
> > +++ b/Documentation/admin-guide/sysctl/kernel.rst
> > @@ -267,6 +267,17 @@ domain names are in general different. For a detailed discussion
> >   see the ``hostname(1)`` man page.
> >   
> >   
> > +export_pmu_events (arm64 only)
> > +==============================
> > +
> > +Controls the PMU export bit (PMCR_EL0.X), which enables the exporting of
> > +events over an IMPLEMENTATION DEFINED PMU event export bus to another device.
> > +
> > +0: disables exporting of events (default).
> > +
> > +1: enables exporting of events.
> > +
> > +
> >   firmware_config
> >   ===============
> >   
> > diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> > index cb69ff1..a8c32a0 100644
> > --- a/arch/arm64/kernel/perf_event.c
> > +++ b/arch/arm64/kernel/perf_event.c
> > @@ -298,6 +298,7 @@ PMU_FORMAT_ATTR(long, "config1:0");
> >   PMU_FORMAT_ATTR(rdpmc, "config1:1");
> >   
> >   static int sysctl_perf_user_access __read_mostly;
> > +static int sysctl_export_pmu_events __read_mostly;
> >   
> >   static inline bool armv8pmu_event_is_64bit(struct perf_event *event)
> >   {
> > @@ -1047,6 +1048,9 @@ static void armv8pmu_reset(void *info)
> >   	if (armv8pmu_has_long_event(cpu_pmu))
> >   		pmcr |= ARMV8_PMU_PMCR_LP;
> >   
> > +	if (sysctl_export_pmu_events)
> > +		pmcr |= ARMV8_PMU_PMCR_X;
> I think we need to do this in armv8pmu_start() rather than armv8pmu_reset(),
> otherwise any changes to the sysctl at runtime won't take effect unless you
> do something like re-online the CPU.
>
> Will

Hi Will ,

We think it may not work.   Say for example pmu->reset is called which 
disables the exporting of events,
and pmu->start not called. This would lead to missing of events.

How about enabling it in both reset and start functions? do you see any 
issue?

--Srinivas