[v9,01/13] Documentation: x86: Add AMD Hardware Feedback Interface documentation

diff --git a/Documentation/arch/x86/amd-hfi.rst b/Documentation/arch/x86/amd-hfi.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+======================================================================
+Hardware Feedback Interface For Hetero Core Scheduling On AMD Platform
+======================================================================
+
+:Copyright: 2025 Advanced Micro Devices, Inc. All Rights Reserved.
+
+:Author: Perry Yuan <perry.yuan@amd.com>
+:Author: Mario Limonciello <mario.limonciello@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: performance-oriented *classic cores* and power-efficient
+*dense cores*. As such, 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.
+
+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 |
++----------+----------------+-------------------------------+---------------------+---------+
+
+Thread classification is performed by the hardware each time that the thread is switched out.
+Threads that don't meet any hardware specified criteria will be classified as "default".
+
+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 following events trigger a table update:
+    * 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
+-------------
+The ranking table is a shared memory region that is used to communicate the
+performance and energy efficiency capabilities of each CPU in the system.
+
+The ranking table design includes rankings for each APIC ID in the system and
+rankings both for performance and efficiency for each workload classification.
+
+.. kernel-doc:: drivers/platform/x86/amd/hfi/hfi.c
+   :doc: amd_shmem_info
+
+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 8ac64d7de4dc9..fb167e6bb9d52 100644
--- a/Documentation/arch/x86/index.rst
+++ b/Documentation/arch/x86/index.rst
@@ -27,6 +27,7 @@  x86-specific Documentation
    intel_txt
    amd-memory-encryption
    amd_hsmp
+   amd-hfi
    tdx
    pti
    mds