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

[Ben Horgan]

Hi Zeng,

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.

> 
> 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.
> 
> Fixes: 6789fb99282c ("arm_mpam: resctrl: Add CDP emulation")
> Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>
> ---
>  drivers/resctrl/mpam_resctrl.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> 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;

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;

Thanks,

Ben


>  
>  	if (enable) {
>  		if (mpam_partid_max < 1)