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+PSCI Performance Measurement
+============================
+
+TF-A provides two instrumentation tools for performing analysis of the PSCI
+implementation:
+
+* PSCI STAT
+* Runtime Instrumentation
+
+This page explains how they may be enabled and used to perform all varieties of
+analysis.
+
+Performance Measurement Framework
+---------------------------------
+
+The Performance Measurement Framework `PMF`_ is a framework that provides
+mechanisms for collecting and retrieving timestamps at runtime from the
+Performance Measurement Unit (`PMU`_). The PMU is a generalized abstraction for
+accessing CPU hardware registers used to measure hardware events. This means,
+for instance, that the PMU might be used to place instrumentation points at
+logical locations in code for tracing purposes.
+
+TF-A utilises the PMF as a backend for the two instrumentation services it
+provides--PSCI Statistics and Runtime Instrumentation. The PMF is used by
+these services to facilitate collection and retrieval of timestamps.  For
+instance, the PSCI Statistics service registers the PMF service
+``psci_svc`` to track its residency statistics.
+
+This is reserved a unique ID, name, and space in memory by the PMF. The
+framework provides a convenient interface for PSCI Statistics to retrieve
+values from ``psci_svc`` at runtime.  Alternatively, the service may be
+configured such that the PMF dumps those values to the console. A platform may
+choose to expose SMCs that allow retrieval of these timestamps from the
+service.
+
+This feature is enabled with the Boolean flag ``ENABLE_PMF``.
+
+PSCI Statistics
+---------------
+
+PSCI Statistics is a runtime service that provides residency statistics for
+power states used by the platform. The service tracks residency time and
+entry count. Residency time is the total time spent in a particular power
+state by a PE. The entry count is the number of times the PE has entered
+the power state. PSCI Statistics implements the optional functions
+``PSCI_STAT_RESIDENCY`` and ``PSCI_STAT_COUNT`` from the `PSCI`_
+specification.
+
+
+.. c:macro:: PSCI_STAT_RESIDENCY
+
+    :param target_cpu: Contains copy of affinity fields in the MPIDR register
+      for identifying the target core (See section 5.1.4 of `PSCI`_
+      specifications for more details).
+    :param power_state: identifier for a specific local
+      state. Generally, this parameter takes the same form as the power_state
+      parameter described for CPU_SUSPEND in section 5.4.2.
+
+    :returns: Time spent in ``power_state``, in microseconds, by ``target_cpu``
+      and the highest level expressed in ``power_state``.
+
+
+.. c:macro:: PSCI_STAT_COUNT
+
+    :param target_cpu: follows the same format as ``PSCI_STAT_RESIDENCY``.
+    :param power_state: follows the same format as ``PSCI_STAT_RESIDENCY``.
+
+    :returns: Number of times the state expressed in ``power_state`` has been
+      used by ``target_cpu`` and the highest level expressed in
+      ``power_state``.
+
+The implementation provides residency statistics only for low power states,
+and does this regardless of the entry mechanism into those states. The
+statistics it collects are set to 0 during shutdown or reset.
+
+PSCI Statistics is enabled with the Boolean build flag
+``ENABLE_PSCI_STAT``.  All Arm platforms utilise the PMF unless another
+collection backend is provided (``ENABLE_PMF`` is implicitly enabled).
+
+Runtime Instrumentation
+-----------------------
+
+The Runtime Instrumentation Service is an instrumentation tool that wraps
+around the PMF to provide timestamp data. Although the service is not
+restricted to PSCI, it is used primarily in TF-A to quantify the total time
+spent in the PSCI implementation. The tool can be used to instrument other
+components in TF-A as well. It is enabled with the Boolean flag
+``ENABLE_RUNTIME_INSTRUMENTATION``, and as with PSCI STAT, requires PMF to
+be enabled.
+
+In PSCI, this service provides instrumentation points in the
+following code paths:
+
+* Entry into the PSCI SMC handler
+* Exit from the PSCI SMC handler
+* Entry to low power state
+* Exit from low power state
+* Entry into cache maintenance operations in PSCI
+* Exit from cache maintenance operations in PSCI
+
+The service captures the cycle count, which allows for the time spent in the
+implementation to be calculated, given the frequency counter.
+
+PSCI SMC Handler Instrumentation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The timestamp during entry into the handler is captured as early as possible
+during the runtime exception, prior to entry into the handler itself. All
+timestamps are stored in memory for later retrieval. The exit timestamp is
+captured after normal return from the PSCI SMC handler, or, if a low power state
+was requested, it is captured in the warm boot path.
+
+*Copyright (c) 2023, Arm Limited. All rights reserved.*
+
+.. _PMF: ../design/firmware-design.html#performance-measurement-framework
+.. _PMU: performance-monitoring-unit.html
+.. _PSCI: https://developer.arm.com/documentation/den0022/latest/