[PATCH v19 11/20] x86/resctrl: Allocate a new bit in union mon_data_bits

[Tony Luck]

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.

Arbitrarily choose the "evtid" field to sacrifice one bit to make
space for a new bit. 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.

The stolen bit is given 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 l3_cache_id (instead of a domain id) to find which
domains must be summed.

Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 arch/x86/kernel/cpu/resctrl/internal.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
index d8156d22cbdc..7957fc38b71c 100644
--- a/arch/x86/kernel/cpu/resctrl/internal.h
+++ b/arch/x86/kernel/cpu/resctrl/internal.h
@@ -133,14 +133,20 @@ struct mon_evt {
  *                     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
+ * @sum:               Set when event must be summed across multiple
+ *                     domains.
+ * @domid:             When @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;
 };
-- 
2.45.0