[PATCH v6 0/3] x86: Capability bits fix and required bits sanity check

[PATCH v6 0/3] x86: Capability bits fix and required bits sanity check

[Maciej Wieczor-Retman]

Series aims to fix the inconsistency between the cpuinfo behavior and
the documentation. Specifically the features that are not compiled are
still present in the cpuinfo bitmasks as enabled. This is not in line
with the documentation which specifies that not-compiled features are
not present in /proc/cpuinfo.

Along adding the disabled feature bitmask initializer array, the
complementary required bitmask initializer is also added. It can be used
to provide a sanity check, after the cpu identification is finished, to
make sure every required bit is set in the final bitmask. A warning with
the cpu number and all required bits that were not set is emitted in
case of the sanity check failure.

Before adding the sanity check a small cleanup can be done. Three places
open code an operation that retrieves either a feature string or, if the
string is not present, the feature number in word:bit format. One of
these places also doesn't check whether the string is actually there or
not. The cleanup patch fixes that and simplifies the other two
instances.

Previous patchset versions:
v5: https://lore.kernel.org/all/cover.1770908783.git.m.wieczorretman@pm.me/
v4: https://lore.kernel.org/all/20250724125346.2792543-1-maciej.wieczor-retman@intel.com/
v3: https://lore.kernel.org/all/20250724094554.2153919-1-maciej.wieczor-retman@intel.com/
v2: https://lore.kernel.org/all/20250723092250.3411923-1-maciej.wieczor-retman@intel.com/
v1: https://lore.kernel.org/all/20250722074439.4069992-1-maciej.wieczor-retman@intel.com/

Maciej Wieczor-Retman (3):
  x86/cpu: Clear feature bits disabled at compile-time
  x86/cpu: Check if feature string is non-zero
  x86/cpu: Do a sanity check on required feature bits

 arch/x86/kernel/cpu/common.c       | 62 +++++++++++++++++++++++++++---
 arch/x86/kernel/cpu/cpuid-deps.c   | 20 ++--------
 arch/x86/tools/cpufeaturemasks.awk |  6 +++
 include/linux/cpu.h                |  2 +
 4 files changed, 67 insertions(+), 23 deletions(-)

-- 
2.53.0

[PATCH v6 1/3] x86/cpu: Clear feature bits disabled at compile-time

[Maciej Wieczor-Retman]

From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>

If some config options are disabled during compile time, they still are
enumerated in macros that use the x86_capability bitmask - cpu_has() or
this_cpu_has().

The features are also visible in /proc/cpuinfo even though they are not
enabled - which is contrary to what the documentation states about the
file. Examples of such feature flags are lam, fred, sgx, ibrs_enhanced,
split_lock_detect, user_shstk, avx_vnni and enqcmd.

Once the cpu_caps_cleared array is initialized with the autogenerated
disabled bitmask apply_forced_caps() will clear the corresponding bits
in boot_cpu_data.x86_capability[] and other secondary cpus'
cpu_data.x86_capability[]. Thus features disabled at compile time won't
show up in /proc/cpuinfo.

Reported-by: Farrah Chen <farrah.chen@intel.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220348
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Cc: <stable@vger.kernel.org> # 6.18.x
---
Changelog v6:
- Remove patch message portions that are not just describing the diff.

 arch/x86/kernel/cpu/common.c       | 3 ++-
 arch/x86/tools/cpufeaturemasks.awk | 6 ++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index e7ab22fce3b5..8d12c5722245 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -732,7 +732,8 @@ static const char *table_lookup_model(struct cpuinfo_x86 *c)
 }
 
 /* Aligned to unsigned long to avoid split lock in atomic bitmap ops */
-__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
+__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long)) =
+	DISABLED_MASK_INITIALIZER;
 __u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
 
 #ifdef CONFIG_X86_32
diff --git a/arch/x86/tools/cpufeaturemasks.awk b/arch/x86/tools/cpufeaturemasks.awk
index 173d5bf2d999..b7f4e775a365 100755
--- a/arch/x86/tools/cpufeaturemasks.awk
+++ b/arch/x86/tools/cpufeaturemasks.awk
@@ -82,6 +82,12 @@ END {
 		}
 		printf " 0\t\\\n";
 		printf "\t) & (1U << ((x) & 31)))\n\n";
+
+		printf "\n#define %s_MASK_INITIALIZER\t\t\t\\", s;
+		printf "\n\t{\t\t\t\t\t\t\\";
+		for (i = 0; i < ncapints; i++)
+			printf "\n\t\t%s_MASK%d,\t\t\t\\", s, i;
+		printf "\n\t}\n\n";
 	}
 
 	printf "#endif /* _ASM_X86_CPUFEATUREMASKS_H */\n";
-- 
2.53.0

[PATCH v6 2/3] x86/cpu: Check if feature string is non-zero

[Maciej Wieczor-Retman]

From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>

In filter_cpuid_features, x86_cap_flags[] is read, but it's not verified
whether the string is non-zero which could lead to unwanted output.

In two more places there are open coded paths that try to retrieve a
feature string, and if there isn't one, the feature word and bit are
returned instead. While correcting filter_cpuid_features() with a helper
it's trivial to also clean up these open coded cases.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
---
Changelog v6:
- Remove parts of the patch message that are redundant and just copy
  what's visible in the diff.
- Redo the helper to use an external char buffer instead of a local
  static string.

 arch/x86/kernel/cpu/common.c     | 26 +++++++++++++++++++++-----
 arch/x86/kernel/cpu/cpuid-deps.c | 20 +++-----------------
 include/linux/cpu.h              |  2 ++
 3 files changed, 26 insertions(+), 22 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 8d12c5722245..44ae2f936395 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -675,6 +675,7 @@ cpuid_dependent_features[] = {
 static void filter_cpuid_features(struct cpuinfo_x86 *c, bool warn)
 {
 	const struct cpuid_dependent_feature *df;
+	char feature_buf[16];
 
 	for (df = cpuid_dependent_features; df->feature; df++) {
 
@@ -697,7 +698,7 @@ static void filter_cpuid_features(struct cpuinfo_x86 *c, bool warn)
 			continue;
 
 		pr_warn("CPU: CPU feature %s disabled, no CPUID level 0x%x\n",
-			x86_cap_flags[df->feature], df->level);
+			x86_cap_name(df->feature, feature_buf), df->level);
 	}
 }
 
@@ -1631,6 +1632,7 @@ static inline bool parse_set_clear_cpuid(char *arg, bool set)
 
 	while (arg) {
 		bool found __maybe_unused = false;
+		char name_buf[16];
 		unsigned int bit;
 
 		opt = strsep(&arg, ",");
@@ -1651,10 +1653,7 @@ static inline bool parse_set_clear_cpuid(char *arg, bool set)
 					setup_clear_cpu_cap(bit);
 				}
 				/* empty-string, i.e., ""-defined feature flags */
-				if (!x86_cap_flags[bit])
-					pr_cont(" %d:%d\n", bit >> 5, bit & 31);
-				else
-					pr_cont(" %s\n", x86_cap_flags[bit]);
+				pr_cont(" %s\n", x86_cap_name(bit, name_buf));
 
 				taint++;
 			}
@@ -1972,6 +1971,23 @@ static void generic_identify(struct cpuinfo_x86 *c)
 #endif
 }
 
+const char *x86_cap_name(unsigned int bit, char *buf)
+{
+	unsigned int word = bit >> 5;
+	const char *name = NULL;
+
+	if (likely(word < NCAPINTS))
+		name = x86_cap_flags[bit];
+	else if (likely(word < NCAPINTS + NBUGINTS))
+		name = x86_bug_flags[bit - 32 * NCAPINTS];
+
+	if (name)
+		return name;
+
+	snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
+	return buf;
+}
+
 /*
  * This does the hard work of actually picking apart the CPU stuff...
  */
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index 146f6f8b0650..7fad18e7d2eb 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -2,6 +2,7 @@
 #include <linux/kernel.h>
 #include <linux/init.h>
 #include <linux/module.h>
+#include <linux/cpu.h>
 #include <asm/cpufeature.h>
 
 struct cpuid_dep {
@@ -156,21 +157,6 @@ void setup_clear_cpu_cap(unsigned int feature)
 	do_clear_cpu_cap(NULL, feature);
 }
 
-/*
- * Return the feature "name" if available, otherwise return
- * the X86_FEATURE_* numerals to make it easier to identify
- * the feature.
- */
-static const char *x86_feature_name(unsigned int feature, char *buf)
-{
-	if (x86_cap_flags[feature])
-		return x86_cap_flags[feature];
-
-	snprintf(buf, 16, "%d*32+%2d", feature / 32, feature % 32);
-
-	return buf;
-}
-
 void check_cpufeature_deps(struct cpuinfo_x86 *c)
 {
 	char feature_buf[16], depends_buf[16];
@@ -185,8 +171,8 @@ void check_cpufeature_deps(struct cpuinfo_x86 *c)
 			 */
 			pr_warn_once("x86 CPU feature dependency check failure: CPU%d has '%s' enabled but '%s' disabled. Kernel might be fine, but no guarantees.\n",
 				     smp_processor_id(),
-				     x86_feature_name(d->feature, feature_buf),
-				     x86_feature_name(d->depends, depends_buf));
+				     x86_cap_name(d->feature, feature_buf),
+				     x86_cap_name(d->depends, depends_buf));
 		}
 	}
 }
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 487b3bf2e1ea..30e5a92430da 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -229,4 +229,6 @@ static inline bool cpu_attack_vector_mitigated(enum cpu_attack_vectors v)
 #define smt_mitigations SMT_MITIGATIONS_OFF
 #endif
 
+const char *x86_cap_name(unsigned int bit, char *buf);
+
 #endif /* _LINUX_CPU_H_ */
-- 
2.53.0

[PATCH v6 3/3] x86/cpu: Do a sanity check on required feature bits

[Maciej Wieczor-Retman]

From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>

After cpu identification concludes, do a sanity check by comparing the
final x86_capability bitmask with the pre-defined required feature bits.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Acked-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
Changelog v6:
- Add Peter's acked-by tag.
- Rename patch subject to imperative form.
- Add a char buffer to the x86_cap_name() call.

 arch/x86/kernel/cpu/common.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 44ae2f936395..f4d4a4c17106 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1988,6 +1988,37 @@ const char *x86_cap_name(unsigned int bit, char *buf)
 	return buf;
 }
 
+/*
+ * As a sanity check compare the final x86_capability bitmask with the initial
+ * predefined required feature bits. In case of a mismatch emit a warning with
+ * the faulty bitmask value.
+ */
+static void verify_required_features(const struct cpuinfo_x86 *c)
+{
+	u32 missing[NCAPINTS] = REQUIRED_MASK_INITIALIZER;
+	char cap_buf[16];
+	u32 error = 0;
+	unsigned int i;
+
+	for (i = 0; i < NCAPINTS; i++) {
+		missing[i] &= ~c->x86_capability[i];
+		error |= missing[i];
+	}
+
+	if (!error)
+		return;		/* All good */
+
+	/*
+	 * At least one required feature is missing. Print a warning,
+	 * and taint the kernel.
+	 */
+	pr_warn("cpu %d: missing required feature(s):", c->cpu_index);
+	for_each_set_bit(i, (void *)missing, NCAPINTS << 5)
+		pr_cont(" %s", x86_cap_name(i, cap_buf));
+	pr_cont("\n");
+	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+}
+
 /*
  * This does the hard work of actually picking apart the CPU stuff...
  */
@@ -2117,6 +2148,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	mcheck_cpu_init(c);
 
 	numa_add_cpu(smp_processor_id());
+
+	verify_required_features(c);
 }
 
 /*
-- 
2.53.0

Re: [PATCH v6 2/3] x86/cpu: Check if feature string is non-zero

[kernel test robot]

Hi Maciej,

kernel test robot noticed the following build warnings:

[auto build test WARNING on tip/x86/core]
[also build test WARNING on tip/smp/core v6.19]
[cannot apply to peterz-queue/sched/core linus/master next-20260219]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Maciej-Wieczor-Retman/x86-cpu-Clear-feature-bits-disabled-at-compile-time/20260220-204916
base:   tip/x86/core
patch link:    https://lore.kernel.org/r/bc0eebc8310acfae28a73e3780a67d721c739b89.1771590895.git.m.wieczorretman%40pm.me
patch subject: [PATCH v6 2/3] x86/cpu: Check if feature string is non-zero
config: x86_64-randconfig-003-20260220 (https://download.01.org/0day-ci/archive/20260221/202602210027.MJpPrtwD-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260221/202602210027.MJpPrtwD-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202602210027.MJpPrtwD-lkp@intel.com/

All warnings (new ones prefixed by >>):

   arch/x86/kernel/cpu/common.c: In function 'x86_cap_name':
>> arch/x86/kernel/cpu/common.c:1987:29: warning: argument to 'sizeof' in 'snprintf' call is the same expression as the destination; did you mean to provide an explicit length? [-Wsizeof-pointer-memaccess]
    1987 |         snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
         |                             ^
>> arch/x86/kernel/cpu/common.c:1987:37: warning: '%u' directive output may be truncated writing between 1 and 9 bytes into a region of size 8 [-Wformat-truncation=]
    1987 |         snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
         |                                     ^~
   arch/x86/kernel/cpu/common.c:1987:36: note: directive argument in the range [0, 134217727]
    1987 |         snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
         |                                    ^~~~~~~
   arch/x86/kernel/cpu/common.c:1987:36: note: directive argument in the range [0, 31]
   arch/x86/kernel/cpu/common.c:1987:9: note: 'snprintf' output between 4 and 13 bytes into a destination of size 8
    1987 |         snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


vim +1987 arch/x86/kernel/cpu/common.c

  1973	
  1974	const char *x86_cap_name(unsigned int bit, char *buf)
  1975	{
  1976		unsigned int word = bit >> 5;
  1977		const char *name = NULL;
  1978	
  1979		if (likely(word < NCAPINTS))
  1980			name = x86_cap_flags[bit];
  1981		else if (likely(word < NCAPINTS + NBUGINTS))
  1982			name = x86_bug_flags[bit - 32 * NCAPINTS];
  1983	
  1984		if (name)
  1985			return name;
  1986	
> 1987		snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
  1988		return buf;
  1989	}
  1990	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v6 2/3] x86/cpu: Check if feature string is non-zero

[Maciej Wieczor-Retman]

I'll replace the sizeof(buf) with 16 for the next version.

-- 
Kind regards
Maciej Wieczór-Retman

On 2026-02-21 at 00:19:59 +0800, kernel test robot wrote:
>Hi Maciej,
>
>kernel test robot noticed the following build warnings:
>
>[auto build test WARNING on tip/x86/core]
>[also build test WARNING on tip/smp/core v6.19]
>[cannot apply to peterz-queue/sched/core linus/master next-20260219]
>[If your patch is applied to the wrong git tree, kindly drop us a note.
>And when submitting patch, we suggest to use '--base' as documented in
>https://git-scm.com/docs/git-format-patch#_base_tree_information]
>
>url:    https://github.com/intel-lab-lkp/linux/commits/Maciej-Wieczor-Retman/x86-cpu-Clear-feature-bits-disabled-at-compile-time/20260220-204916
>base:   tip/x86/core
>patch link:    https://lore.kernel.org/r/bc0eebc8310acfae28a73e3780a67d721c739b89.1771590895.git.m.wieczorretman%40pm.me
>patch subject: [PATCH v6 2/3] x86/cpu: Check if feature string is non-zero
>config: x86_64-randconfig-003-20260220 (https://download.01.org/0day-ci/archive/20260221/202602210027.MJpPrtwD-lkp@intel.com/config)
>compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
>reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260221/202602210027.MJpPrtwD-lkp@intel.com/reproduce)
>
>If you fix the issue in a separate patch/commit (i.e. not just a new version of
>the same patch/commit), kindly add following tags
>| Reported-by: kernel test robot <lkp@intel.com>
>| Closes: https://lore.kernel.org/oe-kbuild-all/202602210027.MJpPrtwD-lkp@intel.com/
>
>All warnings (new ones prefixed by >>):
>
>   arch/x86/kernel/cpu/common.c: In function 'x86_cap_name':
>>> arch/x86/kernel/cpu/common.c:1987:29: warning: argument to 'sizeof' in 'snprintf' call is the same expression as the destination; did you mean to provide an explicit length? [-Wsizeof-pointer-memaccess]
>    1987 |         snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
>         |                             ^
>>> arch/x86/kernel/cpu/common.c:1987:37: warning: '%u' directive output may be truncated writing between 1 and 9 bytes into a region of size 8 [-Wformat-truncation=]
>    1987 |         snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
>         |                                     ^~
>   arch/x86/kernel/cpu/common.c:1987:36: note: directive argument in the range [0, 134217727]
>    1987 |         snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
>         |                                    ^~~~~~~
>   arch/x86/kernel/cpu/common.c:1987:36: note: directive argument in the range [0, 31]
>   arch/x86/kernel/cpu/common.c:1987:9: note: 'snprintf' output between 4 and 13 bytes into a destination of size 8
>    1987 |         snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
>         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>vim +1987 arch/x86/kernel/cpu/common.c
>
>  1973
>  1974	const char *x86_cap_name(unsigned int bit, char *buf)
>  1975	{
>  1976		unsigned int word = bit >> 5;
>  1977		const char *name = NULL;
>  1978
>  1979		if (likely(word < NCAPINTS))
>  1980			name = x86_cap_flags[bit];
>  1981		else if (likely(word < NCAPINTS + NBUGINTS))
>  1982			name = x86_bug_flags[bit - 32 * NCAPINTS];
>  1983
>  1984		if (name)
>  1985			return name;
>  1986
>> 1987		snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
>  1988		return buf;
>  1989	}
>  1990
>
>--
>0-DAY CI Kernel Test Service
>https://github.com/intel/lkp-tests/wiki

Re: [PATCH v6 2/3] x86/cpu: Check if feature string is non-zero

[kernel test robot]

Hi Maciej,

kernel test robot noticed the following build warnings:

[auto build test WARNING on tip/x86/core]
[also build test WARNING on tip/smp/core v6.19]
[cannot apply to peterz-queue/sched/core linus/master next-20260220]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Maciej-Wieczor-Retman/x86-cpu-Clear-feature-bits-disabled-at-compile-time/20260220-204916
base:   tip/x86/core
patch link:    https://lore.kernel.org/r/bc0eebc8310acfae28a73e3780a67d721c739b89.1771590895.git.m.wieczorretman%40pm.me
patch subject: [PATCH v6 2/3] x86/cpu: Check if feature string is non-zero
config: x86_64-randconfig-r051-20260221 (https://download.01.org/0day-ci/archive/20260221/202602210429.MpUz6J91-lkp@intel.com/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202602210429.MpUz6J91-lkp@intel.com/

cocci warnings: (new ones prefixed by >>)
>> arch/x86/kernel/cpu/common.c:1987:15-21: ERROR: application of sizeof to pointer

vim +1987 arch/x86/kernel/cpu/common.c

  1973	
  1974	const char *x86_cap_name(unsigned int bit, char *buf)
  1975	{
  1976		unsigned int word = bit >> 5;
  1977		const char *name = NULL;
  1978	
  1979		if (likely(word < NCAPINTS))
  1980			name = x86_cap_flags[bit];
  1981		else if (likely(word < NCAPINTS + NBUGINTS))
  1982			name = x86_bug_flags[bit - 32 * NCAPINTS];
  1983	
  1984		if (name)
  1985			return name;
  1986	
> 1987		snprintf(buf, sizeof(buf), "%u:%u", word, bit & 31);
  1988		return buf;
  1989	}
  1990	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki