Re: [PATCH v1 2/4] fs/resctrl: Only show 'event_filter' files if events are configurable

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

Hi Reinette,

On Mon, Mar 02, 2026 at 03:12:52PM -0800, Reinette Chatre wrote:
> Hi Ben,
> 
> On 2/25/26 12:19 PM, Ben Horgan wrote:
> > When the counter assignment mode is mbm_event resctrl assumes the mbm
> > events are configurable and exposes the 'event_filter' files. These files
> > live at info/L3_MON/event_configs/<event>/event_filter and are used to
> > display and set the event configuration.
> > 
> > ABMC always supports event configuration but for MPAM they are
> > independent. Decouple event configuration from counter assignment by only
> 
> Could you please elaborate what you mean with "independent" here? If event
> configuration is still supported when assignable counter mode is enabled, why
> can event configuration interface not just remain as-is? Could resctrl not

The two features of ABMC that I'm claiming are independent are: firstly,
requiring assignment of a hardware counter to to CTRL_MON/MON group in order to
allow using bandwidth monitoring when there are fewer hardware counters than
possible CTRL_MON/MON groups (num_rmid) and secondly bandwidth type
configuration for the counters.

The first is concerned with which, if any, hardware counter is used per group
and the second with what the counters are counting. To me these as appear as two
things that should be considered separatedly. Is this clearer?

I'm first trying to address the case where event configuration isn't supported
as we haven't currently got support for that in the MPAM driver and supporting
systems with fewer hardware counters than (PARTID, PMG) without unnecessary
limiting the exposed PARTID/PMG. Some MPAM hardware systems only have a single
bandwidth counter.

> display the existing event configuration and if user cannot modify it return
> a failure when user attempts to do so?

I guess it could. Currently in MPAM we just support mbm_total_bytes and so it
would always be 0x1F. Would we want some other way to indicate that it is fixed
rather than trying to change it? However, if we just remove the configuration
files then it seems natural for mbm_total_bytes to just have the same meaning as
it has when BMEC and ABMC are not enabled.

> 
> Reinette
> 

Thanks,

Ben