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Message-ID: <174420029649.31282.6766699753807343972.tip-bot2@tip-bot2>
Date: Wed, 09 Apr 2025 12:04:56 -0000
From: "tip-bot2 for Borislav Petkov (AMD)" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Borislav Petkov (AMD)" <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] Documentation/x86: Zap the subsection letters
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 254a6d14c9c952e8eae0fafd4fed3778721b948e
Gitweb: https://git.kernel.org/tip/254a6d14c9c952e8eae0fafd4fed3778721b948e
Author: Borislav Petkov (AMD) <bp@...en8.de>
AuthorDate: Wed, 09 Apr 2025 13:14:35 +02:00
Committer: Ingo Molnar <mingo@...nel.org>
CommitterDate: Wed, 09 Apr 2025 13:56:52 +02:00
Documentation/x86: Zap the subsection letters
The subsections already have numbering - no need for the letters too.
Zap the latter.
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20250409111435.GEZ_ZWmz3_lkP8S9Lb@fat_crate.local
---
Documentation/arch/x86/cpuinfo.rst | 51 ++++++++++++++++-------------
1 file changed, 29 insertions(+), 22 deletions(-)
diff --git a/Documentation/arch/x86/cpuinfo.rst b/Documentation/arch/x86/cpuinfo.rst
index 7114f34..f80e2a5 100644
--- a/Documentation/arch/x86/cpuinfo.rst
+++ b/Documentation/arch/x86/cpuinfo.rst
@@ -79,8 +79,9 @@ feature flags.
How are feature flags created?
==============================
-a: Feature flags can be derived from the contents of CPUID leaves.
-------------------------------------------------------------------
+Feature flags can be derived from the contents of CPUID leaves
+--------------------------------------------------------------
+
These feature definitions are organized mirroring the layout of CPUID
leaves and grouped in words with offsets as mapped in enum cpuid_leafs
in cpufeatures.h (see arch/x86/include/asm/cpufeatures.h for details).
@@ -89,8 +90,9 @@ cpufeatures.h, and if it is detected at run time, the flags will be
displayed accordingly in /proc/cpuinfo. For example, the flag "avx2"
comes from X86_FEATURE_AVX2 in cpufeatures.h.
-b: Flags can be from scattered CPUID-based features.
-----------------------------------------------------
+Flags can be from scattered CPUID-based features
+------------------------------------------------
+
Hardware features enumerated in sparsely populated CPUID leaves get
software-defined values. Still, CPUID needs to be queried to determine
if a given feature is present. This is done in init_scattered_cpuid_features().
@@ -104,8 +106,9 @@ has only one feature and would waste 31 bits of space in the x86_capability[]
array. Since there is a struct cpuinfo_x86 for each possible CPU, the wasted
memory is not trivial.
-c: Flags can be created synthetically under certain conditions for hardware features.
--------------------------------------------------------------------------------------
+Flags can be created synthetically under certain conditions for hardware features
+---------------------------------------------------------------------------------
+
Examples of conditions include whether certain features are present in
MSR_IA32_CORE_CAPS or specific CPU models are identified. If the needed
conditions are met, the features are enabled by the set_cpu_cap or
@@ -114,8 +117,8 @@ the feature X86_FEATURE_SPLIT_LOCK_DETECT will be enabled and
"split_lock_detect" will be displayed. The flag "ring3mwait" will be
displayed only when running on INTEL_XEON_PHI_[KNL|KNM] processors.
-d: Flags can represent purely software features.
-------------------------------------------------
+Flags can represent purely software features
+--------------------------------------------
These flags do not represent hardware features. Instead, they represent a
software feature implemented in the kernel. For example, Kernel Page Table
Isolation is purely software feature and its feature flag X86_FEATURE_PTI is
@@ -130,8 +133,8 @@ x86_cap/bug_flags[] arrays in kernel/cpu/capflags.c. The names in the
resulting x86_cap/bug_flags[] are used to populate /proc/cpuinfo. The naming
of flags in the x86_cap/bug_flags[] are as follows:
-a: Flags do not appear by default in /proc/cpuinfo
---------------------------------------------------
+Flags do not appear by default in /proc/cpuinfo
+-----------------------------------------------
Feature flags are omitted by default from /proc/cpuinfo as it does not make
sense for the feature to be exposed to userspace in most cases. For example,
@@ -139,8 +142,8 @@ X86_FEATURE_ALWAYS is defined in cpufeatures.h but that flag is an internal
kernel feature used in the alternative runtime patching functionality. So the
flag does not appear in /proc/cpuinfo.
-b: Specify a flag name if absolutely needed
--------------------------------------------
+Specify a flag name if absolutely needed
+----------------------------------------
If the comment on the line for the #define X86_FEATURE_* starts with a
double-quote character (""), the string inside the double-quote characters
@@ -155,25 +158,28 @@ shall override the new naming with the name already used in /proc/cpuinfo.
Flags are missing when one or more of these happen
==================================================
-a: The hardware does not enumerate support for it.
---------------------------------------------------
+The hardware does not enumerate support for it
+----------------------------------------------
+
For example, when a new kernel is running on old hardware or the feature is
not enabled by boot firmware. Even if the hardware is new, there might be a
problem enabling the feature at run time, the flag will not be displayed.
-b: The kernel does not know about the flag.
--------------------------------------------
+The kernel does not know about the flag
+---------------------------------------
+
For example, when an old kernel is running on new hardware.
-c: The kernel disabled support for it at compile-time.
-------------------------------------------------------
+The kernel disabled support for it at compile-time
+--------------------------------------------------
+
For example, if 5-level-paging is not enabled when building (i.e.,
CONFIG_X86_5LEVEL is not selected) the flag "la57" will not show up [#f1]_.
Even though the feature will still be detected via CPUID, the kernel disables
it by clearing via setup_clear_cpu_cap(X86_FEATURE_LA57).
-d: The feature is disabled at boot-time.
-----------------------------------------
+The feature is disabled at boot-time
+------------------------------------
A feature can be disabled either using a command-line parameter or because
it failed to be enabled. The command-line parameter clearcpuid= can be used
to disable features using the feature number as defined in
@@ -186,8 +192,9 @@ disable specific features. The list of parameters includes, but is not limited
to, nofsgsbase, nosgx, noxsave, etc. 5-level paging can also be disabled using
"no5lvl".
-e: The feature was known to be non-functional.
-----------------------------------------------
+The feature was known to be non-functional
+------------------------------------------
+
The feature was known to be non-functional because a dependency was
missing at runtime. For example, AVX flags will not show up if XSAVE feature
is disabled since they depend on XSAVE feature. Another example would be broken
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