[PATCH 5/6] arm64: mpam: Add memory bandwidth usage (MBWU) documentation

[Ben Horgan <ben.horgan@arm.com>]

Memory bandwidth monitoring make uses of MBWU monitors and is now exposed
to the user via resctrl. Add some documentation so the user knows what to
expect.

Co-developed-by: James Morse <james.morse@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
---
Changes since v4:
Add paragraph about the future.

Changes since v5:
Add R-b's from Fenghua and Gavin
---
 Documentation/arch/arm64/mpam.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Documentation/arch/arm64/mpam.rst b/Documentation/arch/arm64/mpam.rst
index 570f51a8d4eb..67fe515ed501 100644
--- a/Documentation/arch/arm64/mpam.rst
+++ b/Documentation/arch/arm64/mpam.rst
@@ -65,6 +65,28 @@ The supported features are:
   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. (The platform doesn't need to support MPAM.)
+
+  Memory bandwidth monitoring makes use of MBWU monitors in each MSC that
+  makes up the L3 group. 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.
+
+  All these restrictions based on L3 cache are due to resctrl, currently, only
+  supporting monitoring at the L3 scope. It is expected that going forward more
+  MBWU monitors can be exposed to the user after support for more monitoring
+  scopes is added to resctrl.
+
 Reporting Bugs
 ==============
 If you are not seeing the counters or controls you expect please share the
-- 
2.43.0