linux-kernel - Re: [PATCH v5 1/7] arm64: perf: Add missing ISB in armv8pmu_enable

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Date:   Thu, 18 Jun 2020 11:50:51 +0100
From:   Alexandru Elisei <alexandru.elisei@....com>
To:     Stephen Boyd <swboyd@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     mark.rutland@....com, Julien Thierry <julien.thierry@....com>,
        Peter Zijlstra <peterz@...radead.org>, maz@...nel.org,
        Jiri Olsa <jolsa@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, catalin.marinas@....com,
        Namhyung Kim <namhyung@...nel.org>, will@...nel.org,
        Julien Thierry <julien.thierry.kdev@...il.com>
Subject: Re: [PATCH v5 1/7] arm64: perf: Add missing ISB in
 armv8pmu_enable_event()

Hi Stephen,

Thank you very much for taking the time to review the patches!

Comments below.

On 6/17/20 9:01 PM, Stephen Boyd wrote:
> 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.

Typo on my part, will fix it.

>
>> were counting, not the one that we've just enabled.
>>
>> 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 have
>> enabled the event.
>>
>> Prevent both situations from happening by adding an ISB just before we
>> enable the event counter.
>>
>> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.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.

That's a good idea, I'll do that.

Thanks,
Alex