Documentation/x86: Document resctrl's new sparse_masks

The documentation mentions that non-contiguous bit masks are not
supported in Intel Cache Allocation Technology (CAT).

Update the documentation on how to determine if sparse bit masks are
allowed in L2 and L3 CAT.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Peter Newman <peternewman@google.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Tested-by: Peter Newman <peternewman@google.com>
Link: https://lore.kernel.org/r/3e9610997164f648e15c5c2e90d4944ce36504fe.1696934091.git.maciej.wieczor-retman@intel.com

Author: Fenghua Yu

Documentation/arch/x86/resctrl.rst

@@ -124,6 +124,13 @@ related to allocation:
 			"P":
 			      Corresponding region is pseudo-locked. No
 			      sharing allowed.
+"sparse_masks":
+		Indicates if non-contiguous 1s value in CBM is supported.
+
+			"0":
+			      Only contiguous 1s value in CBM is supported.
+			"1":
+			      Non-contiguous 1s value in CBM is supported.
 
 Memory bandwidth(MB) subdirectory contains the following files
 with respect to allocation:
@@ -445,12 +452,13 @@ For cache resources we describe the portion of the cache that is available
 for allocation using a bitmask. The maximum value of the mask is defined
 by each cpu model (and may be different for different cache levels). It
 is found using CPUID, but is also provided in the "info" directory of
-the resctrl file system in "info/{resource}/cbm_mask". Intel hardware
+the resctrl file system in "info/{resource}/cbm_mask". Some Intel hardware
 requires that these masks have all the '1' bits in a contiguous block. So
 0x3, 0x6 and 0xC are legal 4-bit masks with two bits set, but 0x5, 0x9
-and 0xA are not.  On a system with a 20-bit mask each bit represents 5%
-of the capacity of the cache. You could partition the cache into four
-equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
+and 0xA are not. Check /sys/fs/resctrl/info/{resource}/sparse_masks
+if non-contiguous 1s value is supported. On a system with a 20-bit mask
+each bit represents 5% of the capacity of the cache. You could partition
+the cache into four equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
 
 Memory bandwidth Allocation and monitoring
 ==========================================