Documentation: arm64: Document the PMU event counting threshold feature

Add documentation for the new Perf event open parameters and
the threshold_max capability file.

Reviewed-by: Anshuman Khandual <[e-mail]>
Reviewed-by: Suzuki K Poulose <[e-mail]>
Acked-by: Namhyung Kim <[e-mail]>
Signed-off-by: James Clark <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

Documentation/arch/arm64/perf.rst

···
    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
+
+Event Counting Threshold
+==========================================
+
+Overview
+--------
+
+FEAT_PMUv3_TH (Armv8.8) permits a PMU counter to increment only on
+events whose count meets a specified threshold condition. For example if
+threshold_compare is set to 2 ('Greater than or equal'), and the
+threshold is set to 2, then the PMU counter will now only increment by
+when an event would have previously incremented the PMU counter by 2 or
+more on a single processor cycle.
+
+To increment by 1 after passing the threshold condition instead of the
+number of events on that cycle, add the 'threshold_count' option to the
+commandline.
+
+How-to
+------
+
+These are the parameters for controlling the feature:
+
+.. list-table::
+   :header-rows: 1
+
+   * - Parameter
+     - Description
+   * - threshold
+     - Value to threshold the event by. A value of 0 means that
+       thresholding is disabled and the other parameters have no effect.
+   * - threshold_compare
+     - | Comparison function to use, with the following values supported:
+       |
+       | 0: Not-equal
+       | 1: Equals
+       | 2: Greater-than-or-equal
+       | 3: Less-than
+   * - threshold_count
+     - If this is set, count by 1 after passing the threshold condition
+       instead of the value of the event on this cycle.
+
+The threshold, threshold_compare and threshold_count values can be
+provided per event, for example:
+
+.. code-block:: sh
+
+  perf stat -e stall_slot/threshold=2,threshold_compare=2/ \
+            -e dtlb_walk/threshold=10,threshold_compare=3,threshold_count/
+
+In this example the stall_slot event will count by 2 or more on every
+cycle where 2 or more stalls happen. And dtlb_walk will count by 1 on
+every cycle where the number of dtlb walks were less than 10.
+
+The maximum supported threshold value can be read from the caps of each
+PMU, for example:
+
+.. code-block:: sh
+
+  cat /sys/bus/event_source/devices/armv8_pmuv3/caps/threshold_max
+
+  0x000000ff
+
+If a value higher than this is given, then opening the event will result
+in an error. The highest possible maximum is 4095, as the config field
+for threshold is limited to 12 bits, and the Perf tool will refuse to
+parse higher values.
+
+If the PMU doesn't support FEAT_PMUv3_TH, then threshold_max will read
+0, and attempting to set a threshold value will also result in an error.
+threshold_max will also read as 0 on aarch32 guests, even if the host
+is running on hardware with the feature.