linux-kernel - [PATCH v4] x86: Clear feature bits disabled at compile-time

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Message-ID: <20250724125346.2792543-1-maciej.wieczor-retman@intel.com>
Date: Thu, 24 Jul 2025 14:53:46 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: 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>,
	Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
	Kyung Min Park <kyung.min.park@...el.com>,
	Tony Luck <tony.luck@...el.com>
Cc: xin3.li@...el.com,
	maciej.wieczor-retman@...el.com,
	Farrah Chen <farrah.chen@...el.com>,
	stable@...r.kernel.org,
	Borislav Petkov <bp@...e.de>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v4] x86: Clear feature bits disabled at compile-time

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.

Add a DISABLED_MASK_INITIALIZER macro that creates an initializer list
filled with DISABLED_MASKx bitmasks.

Initialize the cpu_caps_cleared array with the autogenerated disabled
bitmask.

Fixes: ea4e3bef4c94 ("Documentation/x86: Add documentation for /proc/cpuinfo feature flags")
Reported-by: Farrah Chen <farrah.chen@...el.com>
Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
Cc: <stable@...r.kernel.org>
---
Changelog v4:
- Fix macro name to match with the patch message.
- Add Peter's SoB.

Changelog v3:
- Remove Fixes: tags, keep only one at the point where the documentation
  changed and promised feature bits wouldn't show up if they're not
  enabled.
- Don't use a helper to initialize cpu_caps_cleared, just statically
  initialize it.
- Remove changes to cpu_caps_set.
- Rewrite patch message to account for changes.

Changelog v2:
- Redo the patch to utilize a more generic solution, not just fix the
  LAM and FRED feature bits.
- Note more feature flags that shouldn't be present.
- Add fixes and cc tags.

 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 77afca95cced..a9040038ad9d 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -704,7 +704,8 @@ 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..1eabbc69f50d 100755
--- a/arch/x86/tools/cpufeaturemasks.awk
+++ b/arch/x86/tools/cpufeaturemasks.awk
@@ -84,5 +84,11 @@ END {
 		printf "\t) & (1U << ((x) & 31)))\n\n";
 	}
 
+		printf "\n#define DISABLED_MASK_INITIALIZER\t\t\t\\";
+		printf "\n\t{\t\t\t\t\t\t\\";
+		for (i = 0; i < ncapints; i++)
+			printf "\n\t\tDISABLED_MASK%d,\t\t\t\\", i;
+		printf "\n\t}\n\n";
+
 	printf "#endif /* _ASM_X86_CPUFEATUREMASKS_H */\n";
 }
-- 
2.49.0