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

[Leo Yan]

On Tue, Feb 02, 2021 at 11:24:48PM +0000, Suzuki Kuruppassery Poulose wrote:
> On 2/2/21 4:38 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@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.
> > +
> > +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.
> > +
> > +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
> > +
> 
> To make this case clear, we could change the command from uname to
> something like:
> 
>     perf record -e cs_etm/contextid1,contextid2/u -- vm

Exactly, will refine.

> Otherwise looks good to me.
> 
> With the above fixed,
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>

Thanks,
Leo