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.

Fixes: 6789fb9 ("arm_mpam: resctrl: Add CDP emulation")
Signed-off-by: Zeng Heng <[email protected]>
[ morse: Added comment for existing code, added hunk to fix this bug from
  Ben H ]
Reviewed-by: James Morse <[email protected]>
Signed-off-by: James Morse <[email protected]>

Author: henryZe

drivers/resctrl/mpam_resctrl.c

@@ -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
+	 * configurations will be provided so there is no aliasing problem.
+	 */
 	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;