Re: [PATCH v5 5/5] arm64: mpam: Add memory bandwidth usage (MBWU) documentation

[Fenghua Yu <fenghuay@nvidia.com>]

On 7/7/26 09:24, Ben Horgan wrote:
> 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>

Thanks.

-Fenghua
> ---
> Changes since v4:
> Add paragraph about the future.
> ---
>   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