Documentation: coresight: Add PID tracing description

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.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210206150833.42120-9-leo.yan@linaro.org
Link: https://lore.kernel.org/r/20210211172038.2483517-4-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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
+
 
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