Commit 06c18e28 authored by Leo Yan's avatar Leo Yan Committed by Greg Kroah-Hartman
Browse files

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: default avatarSuzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: default avatarMike Leach <mike.leach@linaro.org>
Signed-off-by: default avatarLeo Yan <leo.yan@linaro.org>
Signed-off-by: default avatarMathieu 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: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 88f11864

Documentation/trace/coresight/coresight.rst

+32 −0
Original line number Diff line number Diff line
@@ -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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