[PATCH v2 01/13] Documentation: x86: Add AMD Hardware Feedback Interface documentation

[Mario Limonciello <mario.limonciello@amd.com>]

From: Perry Yuan <Perry.Yuan@amd.com>

Introduce a new documentation file, `amd_hfi.rst`, which delves into the
implementation details of the AMD Hardware Feedback Interface and its
associated driver, `amd_hfi`. This documentation describes how the
driver provides hint to the OS scheduling which depends on the capability
of core performance and efficiency ranking data.

This documentation describes
* The design of the driver
* How the driver provides hints to the OS scheduling
* How the driver interfaces with the kernel for efficiency ranking data.

Signed-off-by: Perry Yuan <Perry.Yuan@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
 Documentation/arch/x86/amd-hfi.rst | 116 +++++++++++++++++++++++++++++
 Documentation/arch/x86/index.rst   |   1 +
 2 files changed, 117 insertions(+)
 create mode 100644 Documentation/arch/x86/amd-hfi.rst

diff --git a/Documentation/arch/x86/amd-hfi.rst b/Documentation/arch/x86/amd-hfi.rst
new file mode 100644
index 000000000000..351641ce2821
--- /dev/null
+++ b/Documentation/arch/x86/amd-hfi.rst
@@ -0,0 +1,116 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================================================
+Hardware Feedback Interface For Hetero Core Scheduling On AMD Platform
+======================================================================
+
+:Copyright (C) 2024 Advanced Micro Devices, Inc. All Rights Reserved.
+
+:Author: Perry Yuan <perry.yuan@amd.com>
+
+Overview
+--------
+
+AMD Heterogeneous Core implementations are comprised of more than one
+architectural class and CPUs are comprised of cores of various efficiency
+and power capabilities. Power management strategies must be designed to accommodate
+the complexities introduced by incorporating different core types.
+Heterogeneous systems can also extend to more than two architectural classes as well.
+The purpose of the scheduling feedback mechanism is to provide information to
+the operating system scheduler in real time such that the scheduler can direct
+threads to the optimal core.
+
+``Classic cores`` are generally more performant and ``Dense cores`` are generally more
+efficient.
+The goal of AMD's heterogeneous architecture is to attain power benefit by sending
+background thread to the dense cores while sending high priority threads to the classic
+cores. From a performance perspective, sending background threads to dense cores can free
+up power headroom and allow the classic cores to optimally service demanding threads.
+Furthermore, the area optimized nature of the dense cores allows for an increasing
+number of physical cores. This improved core density will have positive multithreaded
+performance impact.
+
+AMD Heterogeneous Core Driver
+-----------------------------
+
+The ``amd_hfi`` driver delivers the operating system a performance and energy efficiency
+capability data for each CPU in the system. The scheduler can use the ranking data
+from the HFI driver to make task placement decisions.
+
+Thread Classification and Ranking Table Interaction
+----------------------------------------------------
+
+The thread classification is used to select into a ranking table that describes
+an efficiency and performance ranking for each classification.
+
+Threads are classified during runtime into enumerated classes. The classes represent
+thread performance/power characteristics that may benefit from special scheduling behaviors.
+The below table depicts an example of thread classification and a preference where a given thread
+should be scheduled based on its thread class. The real time thread classification is consumed
+by the operating system and is used to inform the scheduler of where the thread should be placed.
+
+Thread Classification Example Table
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
++----------+----------------+-------------------------------+---------------------+---------+
+| class ID | Classification | Preferred scheduling behavior | Preemption priority | Counter |
++----------+----------------+-------------------------------+---------------------+---------+
+| 0        | Default        | Performant                    | Highest             |         |
++----------+----------------+-------------------------------+---------------------+---------+
+| 1        | Non-scalable   | Efficient                     | Lowest              | PMCx1A1 |
++----------+----------------+-------------------------------+---------------------+---------+
+| 2        | I/O bound      | Efficient                     | Lowest              | PMCx044 |
++----------+----------------+-------------------------------+---------------------+---------+
+
+
+AMD Hardware Feedback Interface
+--------------------------------
+
+The Hardware Feedback Interface provides to the operating system information
+about the performance and energy efficiency of each CPU in the system. Each
+capability is given as a unit-less quantity in the range [0-255]. A higher
+performance value indicates higher performance capability, and a higher
+efficiency value indicates more efficiency. Energy efficiency and performance
+are reported in separate capabilities in the shared memory based ranking table.
+
+These capabilities may change at runtime as a result of changes in the
+operating conditions of the system or the action of external factors.
+Power Management FW is responsible for detecting events that would require
+a reordering of the performance and efficiency ranking. Table updates would
+happen relatively infrequently and occur on the time scale of seconds or more.
+
+The mechanism used to trigger a table update like below events:
+    * Thermal Stress Events
+    * Silent Compute
+    * Extreme Low Battery Scenarios
+
+The kernel or a userspace policy daemon can use these capabilities to modify
+task placement decisions. For instance, if either the performance or energy
+capabilities of a given logical processor becomes zero, it is an indication that
+the hardware recommends to the operating system to not schedule any tasks on
+that processor for performance or energy efficiency reasons, respectively.
+
+Implementation details for Linux
+--------------------------------
+
+The implementation of threads scheduling consists of the following steps:
+
+1. A thread is spawned and scheduled to the ideal core using the default
+   heterogeneous scheduling policy.
+2. The processor profiles thread execution and assigns an enumerated classification ID.
+   This classification is communicated to the OS via logical processor scope MSR.
+3. During the thread context switch out the operating system consumes the workload(WL)
+   classification which resides in a logical processor scope MSR.
+4. The OS triggers the hardware to clear its history by writing to an MSR,
+   after consuming the WL classification and before switching in the new thread.
+5. If due to the classification, ranking table, and processor availability,
+   the thread is not on its ideal processor, the OS will then consider scheduling
+   the thread on its ideal processor (if available).
+
+Ranking Table update
+---------------------------
+The power management firmware issues an platform interrupt after updating the ranking
+table and is ready for the operating system to consume it. CPUs receive such interrupt
+and read new ranking table from shared memory which PCCT table has provided, then
+``amd_hfi`` driver parse the new table to provide new consume data for scheduling decisions.
+
+
diff --git a/Documentation/arch/x86/index.rst b/Documentation/arch/x86/index.rst
index 8ac64d7de4dc..7f47229f3104 100644
--- a/Documentation/arch/x86/index.rst
+++ b/Documentation/arch/x86/index.rst
@@ -43,3 +43,4 @@ x86-specific Documentation
    features
    elf_auxvec
    xstate
+   amd_hfi
-- 
2.43.0