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Subject: Re: [PATCH v5 1/7] arm64: perf: Add missing ISB in armv8pmu_enable_event()
From:   Stephen Boyd <swboyd@chromium.org>
Cc:     mark.rutland@arm.com, Julien Thierry <julien.thierry@arm.com>,
        Peter Zijlstra <peterz@infradead.org>, maz@kernel.org,
        Jiri Olsa <jolsa@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, catalin.marinas@arm.com,
        Namhyung Kim <namhyung@kernel.org>, will@kernel.org,
        Julien Thierry <julien.thierry.kdev@gmail.com>
To:     Alexandru Elisei <alexandru.elisei@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Date:   Wed, 17 Jun 2020 13:01:07 -0700
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Quoting Alexandru Elisei (2020-06-17 04:38:45)
> Writes to the PMXEVTYPER_EL0 register are not self-synchronising. In
> armv8pmu_enable_event(), the PE can reorder configuring the event type
> after we have enabled the counter and the interrupt. This can lead to an
> interrupt being asserted because the of the previous event type that we

'because the of the' doesn't read properly.

> were counting, not the one that we've just enabled.
>=20
> The same rationale applies to writes to the PMINTENSET_EL1 register. The =
PE
> can reorder enabling the interrupt at any point in the future after we ha=
ve
> enabled the event.
>=20
> Prevent both situations from happening by adding an ISB just before we
> enable the event counter.
>=20
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_even=
t.c
> index 4d7879484cec..ee180b2a5b39 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -605,6 +605,7 @@ static void armv8pmu_enable_event(struct perf_event *=
event)
>          * Enable interrupt for this counter
>          */
>         armv8pmu_enable_event_irq(event);
> +       isb();

Please add a comment before the isb() explaining the situation. Nobody
knows what this is for when reading the code and they don't want to do
git archaeology to figure it out.