docs(amu): add AMU documentation

This change adds some documentation on the AMU and its purpose. This is
expanded on in later patches.

Change-Id: If2834676790938d8da5ea2ceba37b674f6cc0f01
Signed-off-by: Chris Kay <chris.kay@arm.com>

docs/components/activity-monitors.rst

@@ -0,0 +1,15 @@
+Activity Monitors
+=================
+
+FEAT_AMUv1 of the Armv8-A architecture introduces the Activity Monitors
+extension. This extension describes the architecture for the Activity Monitor
+Unit (|AMU|), an optional non-invasive component for monitoring core events
+through a set of 64-bit counters.
+
+When the ``ENABLE_AMU=1`` build option is provided, Trusted Firmware-A sets up
+the |AMU| prior to its exit from EL3, and will save and restore architected
+|AMU| counters as necessary upon suspend and resume.
+
+--------------
+
+*Copyright (c) 2021, Arm Limited. All rights reserved.*

docs/components/index.rst

@@ -7,6 +7,7 @@ Components
    :numbered:
 
    spd/index
+   activity-monitors
    arm-sip-service
    debugfs-design
    exception-handling

docs/global_substitutions.txt

@@ -1,5 +1,7 @@
 .. |AArch32| replace:: :term:`AArch32`
 .. |AArch64| replace:: :term:`AArch64`
+.. |AMU| replace:: :term:`AMU`
+.. |AMUs| replace:: :term:`AMUs <AMU>`
 .. |API| replace:: :term:`API`
 .. |BTI| replace:: :term:`BTI`
 .. |CoT| replace:: :term:`CoT`

docs/glossary.rst

@@ -15,6 +15,10 @@ You can find additional definitions in the `Arm Glossary`_.
    AArch64
       64-bit execution state of the ARMv8 ISA
 
+   AMU
+      Activity Monitor Unit, a hardware monitoring unit introduced by FEAT_AMUv1
+      that exposes CPU core runtime metrics as a set of counter registers.
+
    API
       Application Programming Interface