Re: [PATCH v2 7/7] Documentation: coresight: Add PID tracing description

[Mike Leach]

Hi,

On Tue, 2 Feb 2021 at 16:39, Leo Yan <leo.yan@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  Documentation/trace/coresight/coresight.rst | 37 +++++++++++++++++++++
>  1 file changed, 37 insertions(+)
>
> diff --git a/Documentation/trace/coresight/coresight.rst b/Documentation/trace/coresight/coresight.rst
> index 0b73acb44efa..771558f22938 100644
> --- a/Documentation/trace/coresight/coresight.rst
> +++ b/Documentation/trace/coresight/coresight.rst
> @@ -512,6 +512,43 @@ 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
> +
> +When the kernel is running at EL2 with Virtualization Host Extensions (VHE),
> +perf records CONTEXTIDR_EL2 in the trace data and can be used as PID when
> +decoding; and if the kernel is running at EL1 with nVHE, CONTEXTIDR_EL1 is
> +traced for PID.
> +

Would this introductory paragraph be better if is explained where the
kernel stores the PID for the different levels, then we logically move
on to how to trace this in perf.

e.g:-

"The lernel 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 Virtualisation Host Extensions (VHE), were the
kernel can run at EL2 as a virtualisation host.
In this case the PID value is stored in CONTEXTIDR_EL2.
perf provides PMU options which program the ETM to insert these values
into the trace data."

> +To support tracing PID for the kernel runs at different exception levels,
> +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.
> +
> +The perf tool automatically sets 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 -- uname
> +    or perf record -e cs_etm//u -- uname
> +
> +will always do the "PID" tracing, independent of the kernel EL.
> +

This is telling me that both cs_etm// and cs_etm/contextid/ have the
same effect - trace PID. Is this correct?
If so, then contextid, contextid1 and contextid2 have no effect except
in specific EL2 circumstances.


> +When the kernel is running at EL2 with VHE, if user wants to trace both the
> +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 -- uname
> +
>


Regards

Mike


>  Generating coverage files for Feedback Directed Optimization: AutoFDO
>  ---------------------------------------------------------------------
> --
> 2.25.1
>


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