[PATCH v7 9/9] Documentation: arm64: Document PMU counters access from userspace

[Rob Herring]

From: Raphael Gault <raphael.gault@xxxxxxx>

Add documentation to describe the access to the pmu hardware counters from
userspace.

Signed-off-by: Raphael Gault <raphael.gault@xxxxxxx>
Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
---
v7:
 - Merge into existing arm64 perf.rst
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/perf.rst | 67 +++++++++++++++++++++++++++++++++++-
 1 file changed, 66 insertions(+), 1 deletion(-)

diff --git a/Documentation/arm64/perf.rst b/Documentation/arm64/perf.rst
index b567f177d385..c40fc2adc451 100644
--- a/Documentation/arm64/perf.rst
+++ b/Documentation/arm64/perf.rst
@@ -2,7 +2,10 @@
 
 .. _perf_index:
 
-=====================
+====
+Perf
+====
+
 Perf Event Attributes
 =====================
 
@@ -88,3 +91,65 @@ exclude_host. However when using !exclude_hv there is a small blackout
 window at the guest entry/exit where host events are not captured.
 
 On VHE systems there are no blackout windows.
+
+Perf Userspace PMU Hardware Counter Access
+==========================================
+
+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/torvalds/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c
+.. _tools/lib/perf/tests/test-evsel.c:
+   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/tests/test-evsel.c
-- 
2.27.0