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

Suzuki K Poulose
2021-02-11T12:39:42Z

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@linaro.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@arm.com>