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From: Rob Herring <>
To: Will Deacon <>,
	Catalin Marinas <>,
	Peter Zijlstra <>,
	Ingo Molnar <>,
	Arnaldo Carvalho de Melo <>,
	Jiri Olsa <>
Cc: Mark Rutland <>,
	Ian Rogers <>,
	Alexander Shishkin <>,,,
	Raphael Gault <>,
	Jonathan Cameron <>,
	Namhyung Kim <>,
Subject: [PATCH v3 10/10] Documentation: arm64: Document PMU counters access from userspace
Date: Fri, 11 Sep 2020 15:51:18 -0600	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

From: Raphael Gault <>

Add a documentation file to describe the access to the pmu hardware
counters from userspace

Signed-off-by: Raphael Gault <>
Signed-off-by: Rob Herring <>
  - Update links to test examples

Changes from Raphael's v4:
  - Convert to rSt
  - Update chained event status
  - Add section for heterogeneous systems
 Documentation/arm64/index.rst                 |  1 +
 .../arm64/perf_counter_user_access.rst        | 56 +++++++++++++++++++
 2 files changed, 57 insertions(+)
 create mode 100644 Documentation/arm64/perf_counter_user_access.rst

diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst
index d9665d83c53a..c712a08e7627 100644
--- a/Documentation/arm64/index.rst
+++ b/Documentation/arm64/index.rst
@@ -15,6 +15,7 @@ ARM64 Architecture
+    perf_counter_user_access
diff --git a/Documentation/arm64/perf_counter_user_access.rst b/Documentation/arm64/perf_counter_user_access.rst
new file mode 100644
index 000000000000..e49e141f10cc
--- /dev/null
+++ b/Documentation/arm64/perf_counter_user_access.rst
@@ -0,0 +1,56 @@
+Access to PMU hardware counter from userspace
+The perf userspace tool relies on the PMU to monitor events. It offers an
+abstraction layer over the hardware counters since the underlying
+implementation is cpu-dependent.
+Arm64 allows userspace tools to have access to the registers storing the
+hardware counters' values directly.
+This targets specifically self-monitoring tasks in order to reduce the overhead
+by directly accessing the registers without having to go through the kernel.
+The focus is set on the armv8 pmuv3 which makes sure that the access to the pmu
+registers is enabled and that the userspace has access to the relevant
+information in order to use them.
+In order to have access to the hardware counter it is necessary to open the event
+using the perf tool interface: the sys_perf_event_open syscall returns a fd which
+can subsequently be used with the mmap syscall in order to retrieve a page of
+memory containing information about the event.
+The PMU driver uses this page to expose to the user the hardware counter's
+index and other necessary data. Using this index enables the user to access the
+PMU registers using the `mrs` instruction.
+The userspace access is supported in libperf using the perf_evsel__mmap()
+and perf_evsel__read() functions. See `tools/lib/perf/tests/test-evsel.c`_ for
+an example.
+About heterogeneous systems
+On heterogeneous systems such as big.LITTLE, userspace PMU counter access can
+only be enabled when the tasks are pinned to a homogeneous subset of cores and
+the corresponding PMU instance is opened by specifying the 'type' attribute.
+The use of generic event types is not supported in this case.
+Have a look at `tools/perf/arch/arm64/tests/user-events.c`_ for an example. It
+can be run using the perf tool to check that the access to the registers works
+correctly from userspace:
+.. code-block:: sh
+  perf test -v user
+About chained events
+Chained events are not supported in userspace. If a 64-bit counter is requested,
+userspace access will only be enabled if the underlying counter is 64-bit.
+.. Links
+.. _tools/perf/arch/arm64/tests/user-events.c:

linux-arm-kernel mailing list

  parent reply	other threads:[~2020-09-11 21:54 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-11 21:51 [PATCH v3 0/10] libperf and arm64 userspace counter access support Rob Herring
2020-09-11 21:51 ` [PATCH v3 01/10] arm64: pmu: Add hook to handle pmu-related undefined instructions Rob Herring
2020-09-28 18:26   ` Will Deacon
2020-09-29 13:46     ` Rob Herring
2020-09-29 17:49       ` Will Deacon
2020-09-29 20:46         ` Rob Herring
2020-09-11 21:51 ` [PATCH v3 02/10] arm64: pmu: Add function implementation to update event index in userpage Rob Herring
2020-09-11 21:51 ` [PATCH v3 03/10] arm64: perf: Enable pmu counter direct access for perf event on armv8 Rob Herring
2020-09-11 21:51 ` [PATCH v3 04/10] tools/include: Add an initial math64.h Rob Herring
2020-09-11 21:51 ` [PATCH v3 05/10] libperf: Add libperf_evsel__mmap() Rob Herring
2020-09-18 14:33   ` Jiri Olsa
2020-09-22 15:28     ` Rob Herring
2020-09-22 18:32       ` Jiri Olsa
2020-09-11 21:51 ` [PATCH v3 06/10] libperf: tests: Add support for verbose printing Rob Herring
2020-09-11 21:51 ` [PATCH v3 07/10] libperf: Add support for user space counter access Rob Herring
2020-09-11 21:51 ` [PATCH v3 08/10] libperf: Add arm64 support to perf_mmap__read_self() Rob Herring
2020-09-11 21:51 ` [PATCH v3 09/10] perf: arm64: Add test for userspace counter access on heterogeneous systems Rob Herring
2020-09-11 21:51 ` Rob Herring [this message]
2020-09-12 20:53 ` [PATCH v3 0/10] libperf and arm64 userspace counter access support Jiri Olsa
2020-09-14 14:21   ` Rob Herring
     [not found]     ` <>
2020-09-16  2:50       ` Rob Herring
2020-09-19  7:22         ` Itaru Kitayama
2020-09-22 15:23           ` Rob Herring

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