fs/resctrl: Disallow the software controller when MBM counters are assignable

The software controller requires that there is one MBM counter per monitor
group that is assigned to the event backing the software controller, as per
mba_MBps_event. When mbm_event mode is in use, it is not guaranteed that any
particular event will have an assigned counter.

Currently, only AMD systems support counter assignment, but the MBA delay
is non-linear and so the software controller is never supported anyway. On
MPAM systems, the MBA delay is linear and so the software controller could
be supported. The MPAM driver, unless a need arises, will not support the
'default' mbm_assign_mode and will always use the 'mbm_event' mode for
memory bandwidth monitoring.

Rather than develop a way to guarantee the counter assignment requirements
needed by the software controller, take the pragmatic approach. Don't allow
the software controller to be used at the same time as 'mbm_event' mode. As
MPAM is the only relevant architecture and it will use 'mbm_event' mode
whenever there are assignable MBM counters, for simplicity's sake, don't
allow the software controller when the MBM counters are assignable.

Implement this by failing the mount if the user requests the software
controller, the mba_MBps option, and the MBM counters are assignable.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://lore.kernel.org/20260506082855.3694761-1-ben.horgan@arm.com

1 files changed, 10 insertions, 6 deletions

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 7d800b306056..af2cbab14497 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2516,10 +2516,13 @@ static void mba_sc_domain_destroy(struct rdt_resource *r,
 }
 
 /*
- * MBA software controller is supported only if
- * MBM is supported and MBA is in linear scale,
- * and the MBM monitor scope is the same as MBA
- * control scope.
+ * The MBA software controller is supported only if MBM is supported and MBA is
+ * in linear scale, and the MBM monitor scope is the same as MBA control scope.
+ *
+ * The software controller cannot be supported when the MBM counters are
+ * assignable.  There is no guarantee that MBM counters are assigned to the
+ * event backing the software controller in all monitoring domains of all
+ * monitoring groups.
  */
 static bool supports_mba_mbps(void)
 {
@@ -2528,7 +2531,8 @@ static bool supports_mba_mbps(void)
 
 	return (resctrl_is_mbm_enabled() &&
 		r->alloc_capable && is_mba_linear() &&
-		r->ctrl_scope == rmbm->mon_scope);
+		r->ctrl_scope == rmbm->mon_scope &&
+		!rmbm->mon.mbm_cntr_assignable);
 }
 
 /*
@@ -2943,7 +2947,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param)
 		ctx->enable_cdpl2 = true;
 		return 0;
 	case Opt_mba_mbps:
-		msg = "mba_MBps requires MBM and linear scale MBA at L3 scope";
+		msg = "mba_MBps requires MBM (mbm_event mode not supported) and linear scale MBA at L3 scope";
 		if (!supports_mba_mbps())
 			return invalfc(fc, msg);
 		ctx->enable_mba_mbps = true;