[PATCH v5 41/41] arm64: mpam: Add initial MPAM documentation

Ben Horgan
2026-02-24T18:01:23Z

MPAM (Memory Partitioning and Monitoring) is now exposed to user-space via
resctrl. Add some documentation so the user knows what features to expect.

Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Signed-off-by: James Morse <james.morse@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes by Ben:
Some tidying, update for current heuristics

Changes from v4:
Fix unusual indentation
---
Documentation/arch/arm64/index.rst | 1 +
Documentation/arch/arm64/mpam.rst | 94 ++++++++++++++++++++++++++++++
2 files changed, 95 insertions(+)
create mode 100644 Documentation/arch/arm64/mpam.rst

diff --git a/Documentation/arch/arm64/index.rst b/Documentation/arch/arm64/index.rst
index af52edc8c0ac..98052b4ef4a1 100644
--- a/Documentation/arch/arm64/index.rst
+++ b/Documentation/arch/arm64/index.rst
@@ -23,6 +23,7 @@ ARM64 Architecture
memory
memory-tagging-extension
mops
+ mpam
perf
pointer-authentication
ptdump
diff --git a/Documentation/arch/arm64/mpam.rst b/Documentation/arch/arm64/mpam.rst
new file mode 100644
index 000000000000..6dc3de54ec9a
--- /dev/null
+++ b/Documentation/arch/arm64/mpam.rst
@@ -0,0 +1,94 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====
+MPAM
+====
+
+What is MPAM
+============
+MPAM (Memory Partitioning and Monitoring) is a feature in the CPUs and memory
+system components such as the caches or memory controllers that allow memory
+traffic to be labelled, partitioned and monitored.
+
+Traffic is labelled by the CPU, based on the control or monitor group the
+current task is assigned to using resctrl. Partitioning policy can be set
+using the schemata file in resctrl, and monitor values read via resctrl.
+See Documentation/filesystems/resctrl.rst for more details.
+
+This allows tasks that share memory system resources, such as caches, to be
+isolated from each other according to the partitioning policy (so called noisy
+neighbours).
+
+Supported Platforms
+===================
+Use of this feature requires CPU support, support in the memory system
+components, and a description from firmware of where the MPAM device controls
+are in the MMIO address space. (e.g. the 'MPAM' ACPI table).
+
+The MMIO device that provides MPAM controls/monitors for a memory system
+component is called a memory system component. (MSC).
+
+Because the user interface to MPAM is via resctrl, only MPAM features that are
+compatible with resctrl can be exposed to user-space.
+
+MSC are considered as a group based on the topology. MSC that correspond with
+the L3 cache are considered together, it is not possible to mix MSC between L2
+and L3 to 'cover' a resctrl schema.
+
+The supported features are:
+
+* Cache portion bitmap controls (CPOR) on the L2 or L3 caches. To expose
+ CPOR at L2 or L3, every CPU must have a corresponding CPU cache at this
+ level that also supports the feature. Mismatched big/little platforms are
+ not supported as resctrl's controls would then also depend on task
+ placement.
+
+* Memory bandwidth maximum controls (MBW_MAX) on or after the L3 cache.
+ resctrl uses the L3 cache-id to identify where the memory bandwidth
+ control is applied. For this reason the platform must have an L3 cache
+ with cache-id's supplied by firmware. (It doesn't need to support MPAM.)
+
+ To be exported as the 'MB' schema, the topology of the group of MSC chosen
+ must match the topology of the L3 cache so that the cache-id's can be
+ repainted. For example: Platforms with Memory bandwidth maximum controls
+ on CPU-less NUMA nodes cannot expose the 'MB' schema to resctrl as these
+ nodes do not have a corresponding L3 cache. If the memory bandwidth
+ control is on the memory rather than the L3 then there must be a single
+ global L3 as otherwise it is unknown which L3 the traffic came from.
+
+ When the MPAM driver finds multiple groups of MSC it can use for the 'MB'
+ schema, it prefers the group closest to the L3 cache.
+
+* Cache Storage Usage (CSU) counters can expose the 'llc_occupancy' provided
+ there is at least one CSU monitor on each MSC that makes up the L3 group.
+ Exposing CSU counters from other caches or devices is not supported.
+
+* Memory Bandwidth Usage (MBWU) on or after the L3 cache. resctrl uses the
+ L3 cache-id to identify where the memory bandwidth is measured. For this
+ reason the platform must have an L3 cache with cache-id's supplied by
+ firmware. (It doesn't need to support MPAM.)
+
+ Memory bandwidth monitoring makes use of MBWU monitors in each MSC that
+ makes up the L3 group. If there are more monitors than the maximum number
+ of control and monitor groups, these will be allocated and configured at
+ boot. Otherwise, the monitors will not be usable as they are required to
+ be free running. If the memory bandwidth monitoring is on the memory
+ rather than the L3 then there must be a single global L3 as otherwise it
+ is unknown which L3 the traffic came from.
+
+ To expose 'mbm_total_bytes', the topology of the group of MSC chosen must
+ match the topology of the L3 cache so that the cache-id's can be
+ repainted. For example: Platforms with Memory bandwidth monitors on
+ CPU-less NUMA nodes cannot expose 'mbm_total_bytes' as these nodes do not
+ have a corresponding L3 cache. 'mbm_local_bytes' is not exposed as MPAM
+ cannot distinguish local traffic from global traffic.
+
+Feature emulation
+=================
+MPAM will emulate the Code Data Prioritisation (CDP) feature on all platforms.
+
+Reporting Bugs
+==============
+If you are not seeing the counters or controls you expect please share the
+debug messages produced when enabling dynamic debug and booting with:
+dyndbg="file mpam_resctrl.c +pl"
--
2.43.0