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