Re: arm_mpam: selecting hard or soft MBW_MAX limits

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

Hi Ping,

On 06/11/2026 04:36, Ping Li wrote:
> Hi Ben,
> 
> mbw_max support in the driver always programs MPAMCFG_MBW_MAX without
> setting MPAMCFG_MBW_MAX.HARDLIM, so the limit is always soft: a

Just to clarify. The limit is always soft whenever a soft limit is supported but if it isn't
MPAMCFG_MBW_MAX.HARDLIM is RAO/WI and so even though the driver writes 0 to MPAMCFG_MBW_MAX.HARDLIM
you would still get a hard limit.

> PARTID above its MAX contends with low priority for bandwidth beyond
> the limit, but is not prevented from exceeding it. On our platform we
> would like to be able to select the hard limit, where a PARTID above
> its MAX is throttled even when the system is otherwise idle - soft
> limits let a noisy task exceed its limit and disturb higher-priority
> work when bandwidth contention appears.
> 
> MPAMF_MBW_IDR.MAXLIM reports whether the MSC implements a hard limit,
> a soft limit, or both, but the driver does not read it today.
> 
> Is there a preferred way to expose this choice? The options we see:
> 
>   - a mount option or info flag, if the choice must be global
>   - a per-resource or per-group resctrl knob
>   - selecting hard limits by default whenever MAXLIM reports the MSC
>     supports them

The current plan is that this will be an extra line in the schemata file. Fenghua was looking into
this and there is some discussion here [1]. This relies on the generic schemata changes to have
multiple controls for resources. The same series as linked although there is a rfc v2 now.

[1] https://lore.kernel.org/lkml/e13cf2e7-3956-4069-ac43-c65e874c6109@intel.com/

Thanks,

Ben

> 
> Thanks,
> Ping