linux-kernel - Re: [PATCH v3 8/8] Documentation: coresight: Add PID tracing description

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Message-ID: <de0be10d-3baf-6d3c-0f1d-2a9f45444e84@arm.com>
Date:   Thu, 11 Feb 2021 12:38:21 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Mike Leach <mike.leach@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        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>,
        Al Grant <al.grant@....com>, coresight@...ts.linaro.org,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 8/8] Documentation: coresight: Add PID tracing
 description

On 2/6/21 3:08 PM, Leo Yan wrote:
> After support the PID tracing for the kernel in EL1 or EL2, the usage
> gets more complicated.
> 
> This patch gives description for the PMU formats of contextID configs,
> this can help users to understand how to control the knobs for PID
> tracing when the kernel is in different ELs.
> 
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> ---
>   Documentation/trace/coresight/coresight.rst | 32 +++++++++++++++++++++
>   1 file changed, 32 insertions(+)
> 
> diff --git a/Documentation/trace/coresight/coresight.rst b/Documentation/trace/coresight/coresight.rst
> index 0b73acb44efa..169749efd8d1 100644
> --- a/Documentation/trace/coresight/coresight.rst
> +++ b/Documentation/trace/coresight/coresight.rst
> @@ -512,6 +512,38 @@ The --itrace option controls the type and frequency of synthesized events
>   Note that only 64-bit programs are currently supported - further work is
>   required to support instruction decode of 32-bit Arm programs.
>   
> +2.2) Tracing PID
> +
> +The kernel can be built to write the PID value into the PE ContextID registers.
> +For a kernel running at EL1, the PID is stored in CONTEXTIDR_EL1.  A PE may
> +implement Arm Virtualization Host Extensions (VHE), which the kernel can
> +run at EL2 as a virtualisation host; in this case, the PID value is stored in
> +CONTEXTIDR_EL2.
> +
> +perf provides PMU formats that program the ETM to insert these values into the
> +trace data; the PMU formats are defined as below:
> +
> +  "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.
> +
> +perf will always enable PID tracing at the relevant EL, this is accomplished by
> +automatically enable the "contextid" config - but for EL2 it is possible to make
> +specific adjustments using configs "contextid1" and "contextid2", E.g. if a user
> +wants to trace PIDs for both host and guest, the two configs "contextid1" and
> +"contextid2" can be set at the same time:
> +
> +  perf record -e cs_etm/contextid1,contextid2/u -- vm
> +
>   


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