Re: [PATCH next] arm_mpam: Fix MBA alloc_capable handling

[Ben Horgan]

Hi James,

On 4/17/26 17:07, James Morse wrote:
> Hi Zeng, Ben,
> 
> On 17/04/2026 10:52, Ben Horgan wrote:
>> On 4/13/26 10:00, Zeng Heng wrote:
>>> The resctrl_arch_set_cdp_enabled() is never invoked with RDT_RESOURCE_MBA
>>> as the rid parameter. Consequently,
>>> mpam_resctrl_controls[RDT_RESOURCE_MBA].cdp_enabled always remains false,
>>> making it unreliable for conditional checks.
>>>
>>> The original logic uses this field to determine MBA state, causing a bug
>>> where MBA allocation is permanently disabled after the mount with CDP
>>> option. Remounting without CDP cannot restore the MBA partition capability.
>>
>> Ah, I see. Thanks for spotting this and sending a patch.
> 
> This shape of bug is likely - I've not had access to a platform with bandwidth
> monitors!
> 
> 
>>> Fix by using the cdp_enabled global parameter directly. Enable MBA
>>> partition only when CDP is disabled and the MBA resource class is
>>> present. Disable MBA partition otherwise.
> 
>>> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
>>> index a9938006d0e6..161fb8905e28 100644
>>> --- a/drivers/resctrl/mpam_resctrl.c
>>> +++ b/drivers/resctrl/mpam_resctrl.c
>>> @@ -217,12 +217,10 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
>>>  		l3->mon.num_rmid = resctrl_arch_system_num_rmid_idx();
>>>  
>>>  	/* The mbw_max feature can't hide cdp as it's a per-partid maximum. */
>>> -	if (cdp_enabled && !mpam_resctrl_controls[RDT_RESOURCE_MBA].cdp_enabled)
>>> -		mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = false;
>>> -
>>> -	if (mpam_resctrl_controls[RDT_RESOURCE_MBA].cdp_enabled &&
>>> -	    mpam_resctrl_controls[RDT_RESOURCE_MBA].class)
>>> +	if (!cdp_enabled && mpam_resctrl_controls[RDT_RESOURCE_MBA].class)
>>>  		mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = true;
>>> +	else
>>> +		mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = false;
> 
> My reading of the original:
> |	if (mpam_resctrl_controls[RDT_RESOURCE_MBA].cdp_enabled &&
> |	    mpam_resctrl_controls[RDT_RESOURCE_MBA].class)
> |		mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = true;
> 
> is that it was trying to do this reset - but never running. (as Zeng's commit message
> describes).
> 
> I've spoken to Ben - and the intent here is a bit more complicated...
> 
> if resctrl does start calling:
> | resctrl_arch_set_cdp_enabled(RDT_RESOURCE_MBA, true)
> 
> then we expect to receive two configurations, one for code and one for data, so the
> aliasing controls problem goes away. The above original code is re-enabling MBA in this
> case. It needs to ignore the global 'cdp_enabled' to be immune to the order of
> resctrl_arch_set_cdp_enabled() calls.
> 
> While this might seem odd, as resctrl doesn't do this today - it is a valid enum value,
> and this is in the woolly "are we Xeon shaped" area.
> 
> I think long term we need a list of non-aliasing controls (min and max etc) which need
> disabling when CDP enabled. But re-enabling them when called explicitly makes sense as
> we have to handle those enum value anyway.
> 
> 
>> Isn't it more that the existing checks are correct (if not totally necessary) and guard against
>> resctrl_arch_set_cdp_enabled() being called with RDT_RESOURCE_MBA but that re-enable on unmount is missing?
>> Does just adding this check make sense? As already commented in the function we assume that
>> resctrl_arch_set_cdp_enabled() is only called with enable=false on error or unmount.
>>
>> --- a/drivers/resctrl/mpam_resctrl.c
>> +++ b/drivers/resctrl/mpam_resctrl.c
>> @@ -224,6 +224,9 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
>>             mpam_resctrl_controls[RDT_RESOURCE_MBA].class)
>>                 mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = true;
>>
>> +       if (!cdp_enabled && mpam_resctrl_controls[RDT_RESOURCE_MBA].class)
>> +               mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = true;
>> +
>>         if (enable) {
>>                 if (mpam_partid_max < 1)
>>                         return -EINVAL;
> 
> 
> I'll add a comment above the original code quoted above describing what it does, and
> take your hunk here, (with a comment that its for umount):
> -------------%<-------------
> arm_mpam: resctrl: Fix MBA CDP alloc_capable handling on unmount
> 
> The code to set MBA's alloc_capable to true appears to be trying to
> restore alloc_capable on unmount. This can never work because
> resctrl_arch_set_cdp_enabled() is never invoked with RDT_RESOURCE_MBA
> as the rid parameter. Consequently,
> mpam_resctrl_controls[RDT_RESOURCE_MBA].cdp_enabled always remains false.
> 
> The alloc_capable setting in resctrl_arch_set_cdp_enabled() is to
> re-enable MBA if the caller opts in to separate control values using
> CDP for this resource. This doesn't happen today.
> 
> Add a comment to describe this.
> 
> However a bug remains where MBA allocation is permanently disabled after
> the mount with CDP option. Remounting without CDP cannot restore the MBA
> partition capability.
> 
> Add a check to re-enable MBA when CDP is disabled, which happens on
> unmount.
> ---
> @@ -220,10 +220,18 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool
> enable)
>         if (cdp_enabled && !mpam_resctrl_controls[RDT_RESOURCE_MBA].cdp_enabled)
>                 mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = false;
> 
> +       /*
> +        * if resctrl has attempted to enable CDP on MBA, re-enable MBA as two
if -> If> +        * configurations will provided so there is no aliasing problem.
will -> will be> +        */
>         if (mpam_resctrl_controls[RDT_RESOURCE_MBA].cdp_enabled &&
>             mpam_resctrl_controls[RDT_RESOURCE_MBA].class)
>                 mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = true;
> 
> +       /* On unmount when CDP is disabled, re-enable MBA */
> +       if (!cdp_enabled && mpam_resctrl_controls[RDT_RESOURCE_MBA].class)
> +               mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = true;
> +
>         if (enable) {
>                 if (mpam_partid_max < 1)
>                         return -EINVAL;
> -------------%<-------------
> 
> Ben - do you want a Co-developed by for this?

No need.

The additional comments and updated commit message make sense to me.

Reviewed-by: Ben Horgan <ben.horgan@xxxxxxx>

Thanks,

Ben

> 
> While I'm at it:
> Reviewed-by: James Morse <james.morse@xxxxxxx>
> 
> 
> Thanks,
> 
> James