x86/cpu: Fix a F00F bug warning and clean up surrounding code

[PATCH v4] x86/cpu: Fix a F00F bug warning and clean up surrounding code

Posted by Sohil Mehta 1 week, 3 days ago

On x86 SMP systems with the F00F bug present, the following
(alternatives_patched) warning occurs for each AP:

  WARNING: arch/x86/kernel/cpu/cpuid-deps.c:126 at do_clear_cpu_cap+0xb4/0x110
  Call Trace:
   clear_cpu_cap+0x8/0x10
   init_intel+0x1b/0x4b0
   identify_cpu+0x154/0x750
   identify_secondary_cpu+0x3d/0x90
   start_secondary+0x6b/0xf0
   startup_32_smp+0x151/0x160

X86_BUG_F00F is first cleared in intel_workarounds() and then set for
the affected models. This sequence works fine on the BSP but on AP
bringup, where alternatives have already been patched, clearing the flag
triggers the warning.

There is no technical reason for clearing the flag before setting it. It
is mainly an artifact of introducing the X86_BUG_F00F flag in commit
e2604b49e8a8 ("x86, cpu: Convert F00F bug detection"). Remove the
unnecessary clearing of the flag.

While at it, remove the kernel notification and the surrounding logic to
inform the user about the workaround exactly once. If needed, the
presence of the F00F bug can be determined through /proc/cpuinfo.

Additionally, the F00F bug was the last remaining user of
clear_cpu_bug(). With no users left, get rid of this helper as well.

Co-developed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: Ahmed S. Darwish <darwi@linutronix.de>
---
v4:
 - Combined the 3-patch series into a single patch. (Boris)
v3:
 - Picked up Ahmed's review tag.
---
 arch/x86/include/asm/cpufeature.h |  1 -
 arch/x86/kernel/cpu/intel.c       | 15 +++------------
 2 files changed, 3 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 3ddc1d33399b..90680f978d43 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -125,7 +125,6 @@ static __always_inline bool _static_cpu_has(u16 bit)
 
 #define cpu_has_bug(c, bit)		cpu_has(c, (bit))
 #define set_cpu_bug(c, bit)		set_cpu_cap(c, (bit))
-#define clear_cpu_bug(c, bit)		clear_cpu_cap(c, (bit))
 
 #define static_cpu_has_bug(bit)		static_cpu_has((bit))
 #define boot_cpu_has_bug(bit)		cpu_has_bug(&boot_cpu_data, (bit))
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index f28c0efb7c8f..abb3984336eb 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -388,24 +388,15 @@ __setup("forcepae", forcepae_setup);
 
 static void intel_workarounds(struct cpuinfo_x86 *c)
 {
-#ifdef CONFIG_X86_F00F_BUG
 	/*
 	 * All models of Pentium and Pentium with MMX technology CPUs
 	 * have the F0 0F bug, which lets nonprivileged users lock up the
-	 * system. Announce that the fault handler will be checking for it.
+	 * system. The fault handler always checks for it.
 	 * The Quark is also family 5, but does not have the same bug.
 	 */
-	clear_cpu_bug(c, X86_BUG_F00F);
-	if (c->x86_vfm >= INTEL_FAM5_START && c->x86_vfm < INTEL_QUARK_X1000) {
-		static int f00f_workaround_enabled;
-
+	if (IS_ENABLED(CONFIG_X86_F00F_BUG) &&
+	    (c->x86_vfm >= INTEL_FAM5_START && c->x86_vfm < INTEL_QUARK_X1000))
 		set_cpu_bug(c, X86_BUG_F00F);
-		if (!f00f_workaround_enabled) {
-			pr_notice("Intel Pentium with F0 0F bug - workaround enabled.\n");
-			f00f_workaround_enabled = 1;
-		}
-	}
-#endif
 
 	/*
 	 * SEP CPUID bug: Pentium Pro reports SEP but doesn't have it until
-- 
2.43.0

[tip: x86/cpu] x86/cpu: Fix a F00F bug warning and clean up surrounding code

Posted by tip-bot2 for Sohil Mehta 1 week, 3 days ago

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     87a451161f36a34307ca09aaa66f233d68642b5f
Gitweb:        https://git.kernel.org/tip/87a451161f36a34307ca09aaa66f233d68642b5f
Author:        Sohil Mehta <sohil.mehta@intel.com>
AuthorDate:    Thu, 28 May 2026 11:48:25 -07:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Thu, 28 May 2026 18:41:32 -07:00

x86/cpu: Fix a F00F bug warning and clean up surrounding code

On x86 SMP systems with the F00F bug present, do_clear_cpu_cap()
rightfully warns that the code clears the X86_BUG_F00F flag after
alternatives have been patched.

X86_BUG_F00F is first cleared in intel_workarounds() and then set for
the affected models. This sequence works fine on the BSP but on AP
bringup, where alternatives have already been patched and clearing the
flag there triggers the warning.

There is no technical reason for clearing the flag before setting it. It
is mainly an artifact of introducing the X86_BUG_F00F flag in

  e2604b49e8a8 ("x86, cpu: Convert F00F bug detection").

Remove the unnecessary clearing of the flag.

While at it, remove the kernel notification and the surrounding logic to
inform the user about the workaround exactly once. If needed, the
presence of the F00F bug can be determined through /proc/cpuinfo.

Additionally, the F00F bug was the last remaining user of clear_cpu_bug().
With no users left, get rid of this helper as well.

  [ bp: Massage commit message. ]

Co-developed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ahmed S. Darwish <darwi@linutronix.de>
Link: https://patch.msgid.link/20260528184826.3642051-1-sohil.mehta@intel.com
---
 arch/x86/include/asm/cpufeature.h |  1 -
 arch/x86/kernel/cpu/intel.c       | 15 +++------------
 2 files changed, 3 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 3ddc1d3..90680f9 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -125,7 +125,6 @@ t_no:
 
 #define cpu_has_bug(c, bit)		cpu_has(c, (bit))
 #define set_cpu_bug(c, bit)		set_cpu_cap(c, (bit))
-#define clear_cpu_bug(c, bit)		clear_cpu_cap(c, (bit))
 
 #define static_cpu_has_bug(bit)		static_cpu_has((bit))
 #define boot_cpu_has_bug(bit)		cpu_has_bug(&boot_cpu_data, (bit))
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index f28c0ef..abb3984 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -388,24 +388,15 @@ __setup("forcepae", forcepae_setup);
 
 static void intel_workarounds(struct cpuinfo_x86 *c)
 {
-#ifdef CONFIG_X86_F00F_BUG
 	/*
 	 * All models of Pentium and Pentium with MMX technology CPUs
 	 * have the F0 0F bug, which lets nonprivileged users lock up the
-	 * system. Announce that the fault handler will be checking for it.
+	 * system. The fault handler always checks for it.
 	 * The Quark is also family 5, but does not have the same bug.
 	 */
-	clear_cpu_bug(c, X86_BUG_F00F);
-	if (c->x86_vfm >= INTEL_FAM5_START && c->x86_vfm < INTEL_QUARK_X1000) {
-		static int f00f_workaround_enabled;
-
+	if (IS_ENABLED(CONFIG_X86_F00F_BUG) &&
+	    (c->x86_vfm >= INTEL_FAM5_START && c->x86_vfm < INTEL_QUARK_X1000))
 		set_cpu_bug(c, X86_BUG_F00F);
-		if (!f00f_workaround_enabled) {
-			pr_notice("Intel Pentium with F0 0F bug - workaround enabled.\n");
-			f00f_workaround_enabled = 1;
-		}
-	}
-#endif
 
 	/*
 	 * SEP CPUID bug: Pentium Pro reports SEP but doesn't have it until