linux-kernel - [PATCH v3 4/4] Documentation/x86: Document resctrl's new sparse

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Message-ID: <c42f54af4790785dec25dd537aaea56bfba78a67.1695977733.git.maciej.wieczor-retman@intel.com>
Date:   Fri, 29 Sep 2023 11:02:18 +0200
From:   Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>
Cc:     Peter Newman <peternewman@...gle.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: [PATCH v3 4/4] Documentation/x86: Document resctrl's new sparse_masks

From: Fenghua Yu <fenghua.yu@...el.com>

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.

Mention the file with feature support information is located in
the /sys/fs/resctrl/info/{resource}/ directories and enumerate what
are the possible outputs on file read operation.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Reviewed-by: Peter Newman <peternewman@...gle.com>
Tested-by: Peter Newman <peternewman@...gle.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
---
Changelog v3:
- Added Peter's tested-by and reviewed-by tags.

Changelog v2:
- Change bitmap naming convention to bit mask. (Reinette)

 Documentation/arch/x86/resctrl.rst | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
index cb05d90111b4..4c6421e2aa31 100644
--- a/Documentation/arch/x86/resctrl.rst
+++ b/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
 ==========================================
-- 
2.42.0