[tip: x86/cache] fs/resctrl: Add monitor property 'mbm_cntr_assign_fixed'

[tip-bot2 for Ben Horgan]

The following commit has been merged into the x86/cache branch of tip:

Commit-ID:     ee3d4c81d89c92fbeb65807971ac22b3dfa49220
Gitweb:        https://git.kernel.org/tip/ee3d4c81d89c92fbeb65807971ac22b3dfa49220
Author:        Ben Horgan <ben.horgan@xxxxxxx>
AuthorDate:    Wed, 06 May 2026 09:28:52 +01:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Thu, 07 May 2026 16:29:14 +02:00

fs/resctrl: Add monitor property 'mbm_cntr_assign_fixed'

Commit

  3b497c3f4f04 ("fs/resctrl: Introduce the interface to display monitoring modes")

introduced CONFIG_RESCTRL_ASSIGN_FIXED but left adding the Kconfig
entry until it was necessary. The counter assignment mode is fixed in
MPAM, even when there are assignable counters, and so addressing this
is needed to support MPAM.

To avoid the burden of another Kconfig entry, replace
CONFIG_RESCTRL_ASSIGN_FIXED with a new property in 'struct resctrl_mon',
'mbm_cntr_assign_fixed' to be set by the architecture.

Do not request the architecture to change the counter assignment mode if it
does not support doing so. Provide insight to user space about why such a
request fails.

Signed-off-by: Ben Horgan <ben.horgan@xxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Babu Moger <babu.moger@xxxxxxx>
Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Tested-by: Babu Moger <babu.moger@xxxxxxx>
Link: https://lore.kernel.org/20260506082855.3694761-1-ben.horgan@xxxxxxx
---
 fs/resctrl/monitor.c    | 8 +++++++-
 include/linux/resctrl.h | 2 ++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 916f7a9..5fbcc64 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -1456,7 +1456,7 @@ int resctrl_mbm_assign_mode_show(struct kernfs_open_file *of,
 		else
 			seq_puts(s, "[default]\n");
 
-		if (!IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED)) {
+		if (!r->mon.mbm_cntr_assign_fixed) {
 			if (enabled)
 				seq_puts(s, "default\n");
 			else
@@ -1507,6 +1507,12 @@ ssize_t resctrl_mbm_assign_mode_write(struct kernfs_open_file *of, char *buf,
 	}
 
 	if (enable != resctrl_arch_mbm_cntr_assign_enabled(r)) {
+		if (r->mon.mbm_cntr_assign_fixed) {
+			ret = -EINVAL;
+			rdt_last_cmd_puts("Counter assignment mode is not configurable\n");
+			goto out_unlock;
+		}
+
 		ret = resctrl_arch_mbm_cntr_assign_set(r, enable);
 		if (ret)
 			goto out_unlock;
diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index 06e8c72..73ff522 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -294,6 +294,7 @@ enum resctrl_schema_fmt {
  * @mbm_assign_on_mkdir:	True if counters should automatically be assigned to MBM
  *				events of monitor groups created via mkdir.
  * @mbm_cntr_configurable:	True if assignable counters are configurable.
+ * @mbm_cntr_assign_fixed:	True if the counter assignment mode is fixed.
  */
 struct resctrl_mon {
 	u32			num_rmid;
@@ -302,6 +303,7 @@ struct resctrl_mon {
 	bool			mbm_cntr_assignable;
 	bool			mbm_assign_on_mkdir;
 	bool			mbm_cntr_configurable;
+	bool			mbm_cntr_assign_fixed;
 };
 
 /**