linux-kernel - Re: [PATCH v2 2/7] coresight: etm-perf: Support PID tracing for kernel at EL2

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Date:   Fri, 5 Feb 2021 13:47:57 +0000
From:   Mike Leach <mike.leach@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Daniel Kiss <Daniel.Kiss@....com>,
        Denis Nikitin <denik@...omium.org>,
        Coresight ML <coresight@...ts.linaro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Grant <al.grant@....com>
Subject: Re: [PATCH v2 2/7] coresight: etm-perf: Support PID tracing for
 kernel at EL2

On Tue, 2 Feb 2021 at 23:06, Suzuki K Poulose <suzuki.poulose@....com> wrote:
>
> On 2/2/21 4:38 PM, Leo Yan wrote:
> > From: Suzuki K Poulose <suzuki.poulose@....com>
> >
> > When the kernel is running at EL2, the PID is stored in CONTEXTIDR_EL2.
> > So, tracing CONTEXTIDR_EL1 doesn't give us the pid of the process.
> > Thus we should trace the VMID with VMIDOPT set to trace CONTEXTIDR_EL2
> > instead of CONTEXTIDR_EL1.  Given that we have an existing config
> > option "contextid" and this will be useful for tracing virtual machines
> > (when we get to support virtualization).
> >
> > So instead, this patch extends option CTXTID with an extra bit
> > ETM_OPT_CTXTID2 (bit 15), thus on an EL2 kernel, we will have another
> > bit available for the perf tool: ETM_OPT_CTXTID is for kernel running in
> > EL1, ETM_OPT_CTXTID2 is used when kernel runs in EL2 with VHE enabled.
> >
> > The tool must be backward compatible for users, i.e, "contextid" today
> > traces PID and that should remain the same; for this purpose, the perf
> > tool is updated to automatically set corresponding bit for the
> > "contextid" config, therefore, the user doesn't have to bother which EL
> > the kernel is running.
> >
> >    i.e, perf record -e cs_etm/contextid/u --
> >
> > will always do the "pid" tracing, independent of the kernel EL.
> >
> > The driver parses the format "contextid", which traces CONTEXTIDR_EL1
> > for ETM_OPT_CTXTID (on EL1 kernel) and traces CONTEXTIDR_EL2 for
> > ETM_OPT_CTXTID2 (on EL2 kernel).
> >
> > Besides the enhancement for format "contexid", extra two formats are
> > introduced: "contextid1" and "contextid2".  This considers to support
> > tracing both CONTEXTIDR_EL1 and CONTEXTIDR_EL2 when the kernel is
> > running at EL2.  Finally, the PMU formats are defined as follow:
> >
> >    "contextid1": Available on both EL1 kernel and EL2 kernel.  When the
> >                  kernel is running at EL1, "contextid1" enables the PID
> >               tracing; when the kernel is running at EL2, this enables
> >               tracing the PID of guest applications.
> >
> >    "contextid2": Only usable when the kernel is running at EL2.  When
> >                  selected, enables PID tracing on EL2 kernel.
> >
> >    "contextid":  Will be an alias for the option that enables PID
> >                  tracing.  I.e,
> >                  contextid == contextid1, on EL1 kernel.
> >                  contextid == contextid2, on EL2 kernel.
> >
> > Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> > Cc: Al Grant <al.grant@....com>
> > Cc: Mike Leach <mike.leach@...aro.org>
> > Cc: Leo Yan <leo.yan@...aro.org>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>
> You may add the following line here :
>
> [ Added two config formats: contextid1, contextid2 ]
>
> > Signed-off-by: Leo Yan <leo.yan@...aro.org>
>
> The patch as such looks good to me.
>
> Suzuki

Reviewed-by: Mike Leach <mike.leach@...aro.org>


--
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK