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
---
v6:
- Update the chained event section with attr.config1 details
v2:
- 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| 60 +++
2 files changed, 61 insertions(+)
create mode 100644 Documentation/arm64/perf_counter_user_access.rst
diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst
index 97d65ba12a35..eb7b1cabbf08 100644
--- a/Documentation/arm64/index.rst
+++ b/Documentation/arm64/index.rst
@@ -18,6 +18,7 @@ ARM64 Architecture
memory
memory-tagging-extension
perf
+perf_counter_user_access
pointer-authentication
silicon-errata
sve
diff --git a/Documentation/arm64/perf_counter_user_access.rst
b/Documentation/arm64/perf_counter_user_access.rst
new file mode 100644
index ..a42800e72458
--- /dev/null
+++ b/Documentation/arm64/perf_counter_user_access.rst
@@ -0,0 +1,60 @@
+=
+Access to PMU hardware counter from userspace
+=
+
+Overview
+
+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.
+
+How-to
+--
+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 and 64-bit counters
+
+Chained events are not supported in conjunction with userspace counter
+access. If a 64-bit counter is requested (attr.config1:0), then
+userspace access must also be requested with attr.config1:1 set. This
+will disable counter chaining. The 'pmc_width' in the user page will
+indicate the actual width of the counter which could be only 32-bits
+depending on event and PMU features.
+
+.. Links
+.. _tools/perf/arch/arm64/tests/user-events.c:
+
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c
--
2.27.0
