arm64: mpam: Add initial MPAM documentation

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: Zeng Heng <zengheng4@huawei.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Tested-by: Gavin Shan <gshan@redhat.com>
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Tested-by: Jesse Chick <jessechick@os.amperecomputing.com>
Signed-off-by: James Morse <james.morse@arm.com>

Documentation/arch/arm64/index.rst

@@ -23,6 +23,7 @@ ARM64 Architecture
     memory
     memory-tagging-extension
     mops
+    mpam
     perf
     pointer-authentication
     ptdump

Documentation/arch/arm64/mpam.rst

@@ -0,0 +1,72 @@
+.. 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. There
+  must be no caches between the L3 and the memory so that the two ends of
+  the path have equivalent traffic.
+
+  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.
+
+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"