Re: [RFC PATCH] arm_mpam: resctrl: Separate MPAM domains

[Reinette Chatre]

Hi Ben,

On 9/14/26 9:11 AM, Ben Horgan wrote:
> On 14/09/2026 16:16, Reinette Chatre wrote:
>> On 9/11/26 1:56 AM, Ben Horgan wrote:
>>> On 10/09/2026 19:10, Reinette Chatre wrote:
>>
>>>>
>>>> Do (admittedly crude) guardrails like below capture the existing driver requirements?
>>>>
>>>> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
>>>> index 9d223057953a..dbd06371890c 100644
>>>> --- a/drivers/resctrl/mpam_resctrl.c
>>>> +++ b/drivers/resctrl/mpam_resctrl.c
>>>> @@ -1681,11 +1681,18 @@ mpam_resctrl_alloc_domain(unsigned int cpu, struct mpam_resctrl_res *res)
>>>>  				continue;       // dummy resource
>>>>  
>>>>  			mon_comp = find_component(mon->class, cpu);
>>>> +			if (!mon_comp) {
>>>> +				WARN_ON_ONCE(0);
>>>> +				err = -EFAULT;
>>>> +				goto offline_ctrl_domain;
>>>> +			}
>>>>  			dom->mon_comp[eventid] = mon_comp;
>>>> -			if (mon_comp)
>>>> -				any_mon_comp = mon_comp;
>>>> +			any_mon_comp = mon_comp;
>>>
>>> This first part which ensures that if there is a class for the monitor then a component can always
>>> be found for the given CPU.
>>>
>>>>  		}
>>>> -		if (!any_mon_comp) {
>>>
>>> The any_mon_comp check could still be useful to confirm that if r->mon_capable then there is a
>>> component for at least one of the events.
>>
>> resctrl fs does not handle such scenario. r->mon_capable guides whether the resource
>> supports monitoring and if it is then all domains belonging to the resource is expected to
>> support all monitoring events associated with the resource. resctrl fs will expose all the
>> monitoring event files based on r->mon_capable and which monitoring events are enabled, there
>> is no finer grained support to expose/hide events per domain.
> 
> Yes. I wasn't intending to suggest anything different. If r->mon_capable then the MPAM driver does
> support all enabled events. Maybe I would would have been more to correct to just say if
> r->mon_capable is true we confirm that we can do some monitoring.

ah - I see. Thank you for clarifying. 

>> If any_mon_comp is true while one of the mon_comp is NULL then resctrl will still expose the
>> event to user space and pass attempts to read the data to MPAM driver.
>>
>> Is this a scenario that the MPAM driver needs to support? That is, users will see event files
>> but reading the data will succeed in some domains but fail in others? 
> 
> No, we don't expect the MPAM driver to support any features inconsistently across resctrl domains.
> If the hardware did happen to be mismatched then we sanitize it, in __class_props_mismatch() we
> ensure the class only exposes the features that are common to all the child components.
Thank you very much.

Reinette