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SubjectRe: [PATCH v2 7/7] Documentation: coresight: Add PID tracing description
Hi Mike,

On Wed, Feb 03, 2021 at 05:39:54PM +0000, Mike Leach wrote:

[...]

> > +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."

Will in next spin; thanks a lot for writing up!

> > +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?

Correct.

> If so, then contextid, contextid1 and contextid2 have no effect except
> in specific EL2 circumstances.

Yes, exactly.

Thanks,
Leo

> > +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

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