x86/resctrl: Allocate a new field in union mon_data_bits

When Sub-NUMA Cluster (SNC) mode is enabled, the legacy monitor reporting files
must report the sum of the data from all of the SNC nodes that share the L3
cache that is referenced by the monitor file.

Resctrl squeezes all the attributes of these files into 32 bits so they can be
stored in the "priv" field of struct kernfs_node.

Currently, only three monitor events are defined by enum resctrl_event_id so
reducing it from 8 bits to 7 bits still provides more than enough space to
represent all the known event types.

But note that this choice was arbitrary.  The "rid" field is also far wider
than needed for the current number of resource id types.  This structure is
purely internal to resctrl, no ABI issues with modifying it. Subsequent changes
may rearrange the allocation of bits between each of the fields as needed.

Give the bit to a new "sum" field that indicates that reading this file must
sum across SNC nodes. This bit also indicates that the domid field is the id of
an L3 cache (instead of a domain id) to find which domains must be summed.

Fix up other issues in the kerneldoc description for mon_data_bits.
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://lore.kernel.org/r/20240628215619.76401-13-tony.luck@intel.com

arch/x86/kernel/cpu/resctrl/internal.h

@@ -127,19 +127,25 @@ struct mon_evt {
 };
 
 /**
- * union mon_data_bits - Monitoring details for each event file
+ * union mon_data_bits - Monitoring details for each event file.
  * @priv:              Used to store monitoring event data in @u
- *                     as kernfs private data
- * @rid:               Resource id associated with the event file
- * @evtid:             Event id associated with the event file
- * @domid:             The domain to which the event file belongs
- * @u:                 Name of the bit fields struct
+ *                     as kernfs private data.
+ * @u.rid:             Resource id associated with the event file.
+ * @u.evtid:           Event id associated with the event file.
+ * @u.sum:             Set when event must be summed across multiple
+ *                     domains.
+ * @u.domid:           When @u.sum is zero this is the domain to which
+ *                     the event file belongs. When @sum is one this
+ *                     is the id of the L3 cache that all domains to be
+ *                     summed share.
+ * @u:                 Name of the bit fields struct.
  */
 union mon_data_bits {
 	void *priv;
 	struct {
 		unsigned int rid		: 10;
-		enum resctrl_event_id evtid	: 8;
+		enum resctrl_event_id evtid	: 7;
+		unsigned int sum		: 1;
 		unsigned int domid		: 14;
 	} u;
 };