Messages in this thread | | | Date | Mon, 18 Jul 2022 09:55:32 +0530 | Subject | Re: [PATCH v3] drivers/perf: arm_spe: Fix consistency of SYS_PMSCR_EL1.CX | From | Anshuman Khandual <> |
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On 7/14/22 16:40, Suzuki K Poulose wrote: > On 14/07/2022 07:13, Anshuman Khandual wrote: >> The arm_spe_pmu driver will enable SYS_PMSCR_EL1.CX in order to add CONTEXT >> packets into the traces, if the owner of the perf event runs with required >> capabilities i.e CAP_PERFMON or CAP_SYS_ADMIN via perfmon_capable() helper. >> >> The value of this bit is computed in the arm_spe_event_to_pmscr() function >> but the check for capabilities happens in the pmu event init callback i.e >> arm_spe_pmu_event_init(). This suggests that the value of the CX bit should >> remain consistent for the duration of the perf session. >> >> However, the function arm_spe_event_to_pmscr() may be called later during >> the event start callback i.e arm_spe_pmu_start() when the "current" process >> is not the owner of the perf session, hence the CX bit setting is currently >> not consistent. >> >> One way to fix this, is by caching the required value of the CX bit during >> the initialization of the PMU event, so that it remains consistent for the >> duration of the session. It uses currently unused 'event->hw.flags' element >> to cache perfmon_capable() value, which can be referred during event start >> callback to compute SYS_PMSCR_EL1.CX. This ensures consistent availability >> of context packets in the trace as per event owner capabilities. >> >> Drop BIT(SYS_PMSCR_EL1_CX_SHIFT) check in arm_spe_pmu_event_init(), because >> now CX bit cannot be set in arm_spe_event_to_pmscr() with perfmon_capable() >> disabled. >> >> Cc: Will Deacon <will@kernel.org> >> Cc: Mark Rutland <mark.rutland@arm.com> >> Cc: Alexey Budankov <alexey.budankov@linux.intel.com> >> Cc: linux-arm-kernel@lists.infradead.org >> Cc: linux-kernel@vger.kernel.org >> Fixes: cea7d0d4a59b ("drivers/perf: Open access for CAP_PERFMON privileged process") > > TBH, this is not sufficient. The above commit simply replaced the capable() check with perfmon_capable() wrapper. The "incorrect check > in the wrong task context" existed since : > > d5d9696b0380 ("drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension"). > > So I would recommend using that for the Fixes tag. And any stable > backports without perfmon_capable() could fallback to using capable() > check, like we do in this patch, from the event_init.
Makes sense. I will respin the patch with above "Fixes: " tag unless there is some other feedback here.
> > Otherwise, > > Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com> >
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