linux-kernel - Re: [PATCH v3] drivers/perf: arm_spe: Fix consistency of SYS_PMSCR

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Date:   Thu, 14 Jul 2022 12:10:45 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     german.gomez@....com, james.clark@....com,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] drivers/perf: arm_spe: Fix consistency of
 SYS_PMSCR_EL1.CX

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@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Alexey Budankov <alexey.budankov@...ux.intel.com>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.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.

Otherwise,

Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>