Messages in this thread | | | Date | Wed, 13 Oct 2021 18:30:19 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v10 3/5] arm64: perf: Add userspace counter access disable switch |
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On Tue, Sep 14, 2021 at 03:47:58PM -0500, Rob Herring wrote: > Like x86, some users may want to disable userspace PMU counter > altogether. Add a sysctl 'perf_user_access' file to control userspace > counter access. The default is '0' which is disabled. Writing '1' > enables access. > > Note that x86 also supports writing '2' to globally enable user access.
For clarity it might be worth mentioning that on x86 this is controlled by the PMU's `rdpmc` sysfs attribute, i.e.
Note that x86 supports globally enabling user access by writing '2' to /sys/bus/event_source/devices/cpu/rdpmc
> As there's not existing userspace support to worry about, this shouldn't > be necessary for Arm. It could be added later if the need arises. > > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Arnaldo Carvalho de Melo <acme@kernel.org> > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> > Cc: Jiri Olsa <jolsa@redhat.com> > Cc: Namhyung Kim <namhyung@kernel.org> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: linux-perf-users@vger.kernel.org > Acked-by: Will Deacon <will@kernel.org> > Signed-off-by: Rob Herring <robh@kernel.org> > --- > v10: > - Add documentation > - Use a custom handler (needed on the next patch) > v9: > - Use sysctl instead of sysfs attr > - Default to disabled > v8: > - New patch > --- > Documentation/admin-guide/sysctl/kernel.rst | 11 +++++++++ > arch/arm64/kernel/perf_event.c | 27 +++++++++++++++++++++ > 2 files changed, 38 insertions(+) > > diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst > index 426162009ce9..346a0dba5703 100644 > --- a/Documentation/admin-guide/sysctl/kernel.rst > +++ b/Documentation/admin-guide/sysctl/kernel.rst > @@ -905,6 +905,17 @@ enabled, otherwise writing to this file will return ``-EBUSY``. > The default value is 8. > > > +perf_user_access (arm64 only) > +================================= > + > +Controls user space access for reading perf event counters. When set to 1, > +user space can read performance monitor counter registers directly. > + > +The default value is 0 (access disabled). > + > +See Documentation/arm64/perf.rst for more information.
Looking at the existing perf sysctls:
# ls /proc/sys/kernel/perf* /proc/sys/kernel/perf_cpu_time_max_percent /proc/sys/kernel/perf_event_max_contexts_per_stack /proc/sys/kernel/perf_event_max_sample_rate /proc/sys/kernel/perf_event_max_stack /proc/sys/kernel/perf_event_mlock_kb /proc/sys/kernel/perf_event_paranoid
I see that other than `perf_cpu_time_max_percent`, we've used `perf_event_` as the prefix, and I suspect we should do the same here, but I guess it may not matter either way.
> + > + > pid_max > ======= > > diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c > index b4044469527e..a8f8dd741aeb 100644 > --- a/arch/arm64/kernel/perf_event.c > +++ b/arch/arm64/kernel/perf_event.c > @@ -286,6 +286,8 @@ static const struct attribute_group armv8_pmuv3_events_attr_group = { > PMU_FORMAT_ATTR(event, "config:0-15"); > PMU_FORMAT_ATTR(long, "config1:0"); > > +static int sysctl_perf_user_access __read_mostly; > + > static inline bool armv8pmu_event_is_64bit(struct perf_event *event) > { > return event->attr.config1 & 0x1; > @@ -1104,6 +1106,29 @@ static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu) > return probe.present ? 0 : -ENODEV; > } > > +int armv8pmu_proc_user_access_handler(struct ctl_table *table, int write, > + void *buffer, size_t *lenp, loff_t *ppos) > +{ > + int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos); > + if (ret || !write || sysctl_perf_user_access) > + return ret; > + > + return 0; > +}
Maybe this is needed in the next patch, but the if statement is entirely redundant on this patch and looks really odd.
Can we please either:
1) Use proc_dointvec_minmax() directly in this patch (which is what Will Acked in v9) and add the wrapper in the next patch when we need it.
2) make this:
| int armv8pmu_proc_user_access_handler(struct ctl_table *table, int write, | void *buffer, size_t *lenp, loff_t *ppos) | { | return proc_dointvec_minmax(table, write, buffer, lenp, ppos); | }
... and flesh it out in the next patch.
With either of those two options (and regardless of whether the attribute is renamed):
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Thanks, Mark.
> + > + return 0; > +} > + > +static struct ctl_table armv8_pmu_sysctl_table[] = { > + { > + .procname = "perf_user_access", > + .data = &sysctl_perf_user_access, > + .maxlen = sizeof(unsigned int), > + .mode = 0644, > + .proc_handler = armv8pmu_proc_user_access_handler, > + .extra1 = SYSCTL_ZERO, > + .extra2 = SYSCTL_ONE, > + }, > + { } > +}; > + > static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name, > int (*map_event)(struct perf_event *event), > const struct attribute_group *events, > @@ -1136,6 +1161,8 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name, > cpu_pmu->attr_groups[ARMPMU_ATTR_GROUP_CAPS] = caps ? > caps : &armv8_pmuv3_caps_attr_group; > > + register_sysctl("kernel", armv8_pmu_sysctl_table); > + > return 0; > } > > -- > 2.30.2 >
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