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Documentation/x86: Update the naming of CPU features for /proc/cpuinfo

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Commit:

  78ce84b9e0 ("x86/cpufeatures: Flip the /proc/cpuinfo appearance logic")

changed how CPU feature names should be specified. Update document to
reflect the same.

Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250409111341.GDZ_ZWZS4LckBcirLE@fat_crate.local
This commit is contained in:
Naveen N Rao (AMD) 2025-04-09 13:13:41 +02:00 committed by Ingo Molnar
parent 83f6665a49
commit af76f7d57e
1 changed files with 11 additions and 15 deletions
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26
Documentation/arch/x86/cpuinfo.rst
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@ -130,14 +130,18 @@ 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: The name of the flag is from the string in X86_FEATURE_<name> by default.
----------------------------------------------------------------------------
By default, the flag <name> in /proc/cpuinfo is extracted from the respective
X86_FEATURE_<name> in cpufeatures.h. For example, the flag "avx2" is from
X86_FEATURE_AVX2.
a: 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,
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
-------------------------------------------
b: The naming can be overridden.
--------------------------------
If the comment on the line for the #define X86_FEATURE_* starts with a
double-quote character (""), the string inside the double-quote characters
will be the name of the flags. For example, the flag "sse4_1" comes from
@ -148,14 +152,6 @@ needed. For instance, /proc/cpuinfo is a userspace interface and must remain
constant. If, for some reason, the naming of X86_FEATURE_<name> changes, one
shall override the new naming with the name already used in /proc/cpuinfo.
c: The naming override can be "", which means it will not appear in /proc/cpuinfo.
----------------------------------------------------------------------------------
The feature shall be omitted from /proc/cpuinfo if it does not make sense for
the feature to be exposed to userspace. For example, X86_FEATURE_ALWAYS is
defined in cpufeatures.h but that flag is an internal kernel feature used
in the alternative runtime patching functionality. So, its name is overridden
with "". Its flag will not appear in /proc/cpuinfo.
Flags are missing when one or more of these happen
==================================================