linux-kernel - Re: [PATCH v6 02/10] arm64: perf: Enable PMU counter direct access for perf event

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Message-ID: <20210401090427.GB8781@willie-the-truck>
Date:   Thu, 1 Apr 2021 10:04:28 +0100
From:   Will Deacon <will@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Ian Rogers <irogers@...gle.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Honnappa Nagarahalli <honnappa.nagarahalli@....com>,
        Zachary.Leaf@....com, Raphael Gault <raphael.gault@....com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 02/10] arm64: perf: Enable PMU counter direct access
 for perf event

On Wed, Mar 31, 2021 at 12:52:11PM -0500, Rob Herring wrote:
> On Wed, Mar 31, 2021 at 10:38 AM Will Deacon <will@...nel.org> wrote:
> >
> > On Tue, Mar 30, 2021 at 04:08:11PM -0500, Rob Herring wrote:
> > > On Tue, Mar 30, 2021 at 12:09 PM Rob Herring <robh@...nel.org> wrote:
> > > > On Tue, Mar 30, 2021 at 10:31 AM Will Deacon <will@...nel.org> wrote:
> > > > > The logic here feels like it
> > > > > could with a bit of untangling.
> > > >
> > > > Yes, I don't love it, but couldn't come up with anything better. It is
> > > > complicated by the fact that flags have to be set before we assign the
> > > > counter and can't set/change them when we assign the counter. It would
> > > > take a lot of refactoring with armpmu code to fix that.
> > >
> > > How's this instead?:
> > >
> > > if (armv8pmu_event_want_user_access(event) || !armv8pmu_event_is_64bit(event))
> > >         event->hw.flags |= ARMPMU_EL0_RD_CNTR;
> > >
> > > /*
> > > * At this point, the counter is not assigned. If a 64-bit counter is
> > > * requested, we must make sure the h/w has 64-bit counters if we set
> > > * the event size to 64-bit because chaining is not supported with
> > > * userspace access. This may still fail later on if the CPU cycle
> > > * counter is in use.
> > > */
> > > if (armv8pmu_event_is_64bit(event) &&
> > >     (!armv8pmu_event_want_user_access(event) ||
> > >      armv8pmu_has_long_event(cpu_pmu) || (hw_event_id ==
> > > ARMV8_PMUV3_PERFCTR_CPU_CYCLES)))
> > >         event->hw.flags |= ARMPMU_EVT_64BIT;
> >
> > I thought there were some cases where we could assign cycles event to an
> > event counter; does that not happen anymore?
> 
> Yes, but if we hit that scenario when the user has asked for 64-bit
> user access, then we return an error later when assigning the counter.
> I think we can assume if users have gone to the trouble of requesting
> 64-bit counters, then they can deal with ensuring they don't have
> multiple users.
> 
> Otherwise, the only way I see to simplify this is we only support
> 64-bit counters in userspace when we have v8.5 PMU.

I'm happy to start from that position, and then we can extend it later if
there's a need.

Will