[PATCH 1/2] x86: Don't clear X86_BUG_F00F before setting it

[PATCH 1/2] x86: Don't clear X86_BUG_F00F before setting it

[Richard Weinberger]

On x86 SMP systems with the F00F bug is present the following
warning occurs for each application processor:

[    0.013122] ------------[ cut here ]------------
[    0.013122] WARNING: arch/x86/kernel/cpu/cpuid-deps.c:126 at do_clear_cpu_cap+0xb4/0x110, CPU#1: swapper/1/0
[    0.013122] Modules linked in:
[    0.013122] CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 6.19.0-10445-g0f2acd3148e0 #379 PREEMPT(full)
[    0.013122] Hardware name:  , BIOS 4.51 PG 04/03/97
[    0.013122] EIP: do_clear_cpu_cap+0xb4/0x110
[    0.013122] Code: 08 83 c7 08 85 c0 75 ce 84 d2 75 c2 8b 45 f0 64 2b 05 08 80 af c4 75 64 83 c4 68 5b 5e 5f 5d c3 8b 0d 78 7a a1 c4 85 c9 74 83 <0f> 0b e9 7c ff ff ff 2e 8d 74 26 00 b8 20 79 a1 c4 89 55 8c e8 33
[    0.013122] EAX: 00000000 EBX: c1143ed8 ECX: 00000001 EDX: 000002c0
[    0.013122] ESI: c7ee8060 EDI: c1143f38 EBP: c1143f48 ESP: c1143ed4
[    0.013122] DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068 EFLAGS: 00210002
[    0.013122] CR0: 80050033 CR2: 00000000 CR3: 04b15000 CR4: 00000010
[    0.013122] Call Trace:
[    0.013122]  clear_cpu_cap+0x8/0x10
[    0.013122]  init_intel+0x1b/0x4b0
[    0.013122]  identify_cpu+0x154/0x750
[    0.013122]  identify_secondary_cpu+0x3d/0x90
[    0.013122]  start_secondary+0x6b/0xf0
[    0.013122]  startup_32_smp+0x151/0x160
[    0.013122] ---[ end trace 0000000000000000 ]---

Commit ee8962082a44 ("x86/alternatives: Catch late X86_FEATURE modifiers") introduced the warning
to catch changes to x86 features after alternatives have been applied.

Since the X86_BUG_F00F CPU feature is first cleared in
intel_workarounds() and then set for the affected families, the feature
flags are unnecessarily modified. On systems where the F00F bug is
actually present, clearing the flag first triggers the warning.

Fix this by not clearing the flag first. Clearing it first is relict from
commit e2604b49e8a8 ("x86, cpu: Convert F00F bug detection").

Fixes: ee8962082a44 ("x86/alternatives: Catch late X86_FEATURE modifiers")
Signed-off-by: Richard Weinberger <richard@nod.at>
---
 arch/x86/kernel/cpu/intel.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 646ff33c4651..73f6bf182333 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -394,7 +394,6 @@ static void intel_workarounds(struct cpuinfo_x86 *c)
 	 * system. Announce that the fault handler will be checking 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;
 
-- 
2.51.0

[PATCH 2/2] [RFC] x86: Remove clear_cpu_bug()

[Richard Weinberger]

F00F was the only user of clear_cpu_bug().
With no users left, remove this helper.

Signed-off-by: Richard Weinberger <richard@nod.at>
---
 arch/x86/include/asm/cpufeature.h | 1 -
 1 file changed, 1 deletion(-)

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))
-- 
2.51.0

Re: [PATCH 1/2] x86: Don't clear X86_BUG_F00F before setting it

[Sohil Mehta]

On 2/16/2026 2:43 AM, Richard Weinberger wrote:
> On x86 SMP systems with the F00F bug is present the following
> warning occurs for each application processor:
> 
> [    0.013122] ------------[ cut here ]------------
> [    0.013122] WARNING: arch/x86/kernel/cpu/cpuid-deps.c:126 at do_clear_cpu_cap+0xb4/0x110, CPU#1: swapper/1/0
> [    0.013122] Modules linked in:
> [    0.013122] CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 6.19.0-10445-g0f2acd3148e0 #379 PREEMPT(full)
> [    0.013122] Hardware name:  , BIOS 4.51 PG 04/03/97
> [    0.013122] EIP: do_clear_cpu_cap+0xb4/0x110
> [    0.013122] Code: 08 83 c7 08 85 c0 75 ce 84 d2 75 c2 8b 45 f0 64 2b 05 08 80 af c4 75 64 83 c4 68 5b 5e 5f 5d c3 8b 0d 78 7a a1 c4 85 c9 74 83 <0f> 0b e9 7c ff ff ff 2e 8d 74 26 00 b8 20 79 a1 c4 89 55 8c e8 33
> [    0.013122] EAX: 00000000 EBX: c1143ed8 ECX: 00000001 EDX: 000002c0
> [    0.013122] ESI: c7ee8060 EDI: c1143f38 EBP: c1143f48 ESP: c1143ed4
> [    0.013122] DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068 EFLAGS: 00210002
> [    0.013122] CR0: 80050033 CR2: 00000000 CR3: 04b15000 CR4: 00000010
> [    0.013122] Call Trace:
> [    0.013122]  clear_cpu_cap+0x8/0x10
> [    0.013122]  init_intel+0x1b/0x4b0
> [    0.013122]  identify_cpu+0x154/0x750
> [    0.013122]  identify_secondary_cpu+0x3d/0x90
> [    0.013122]  start_secondary+0x6b/0xf0
> [    0.013122]  startup_32_smp+0x151/0x160
> [    0.013122] ---[ end trace 0000000000000000 ]---
> 

This trace should probably be trimmed. The function call list is likely
more than enough.

https://www.kernel.org/doc/html/latest/process/submitting-patches.html#backtraces-in-commit-messages


> Commit ee8962082a44 ("x86/alternatives: Catch late X86_FEATURE modifiers") introduced the warning
> to catch changes to x86 features after alternatives have been applied.
> 
> Since the X86_BUG_F00F CPU feature is first cleared in
> intel_workarounds() and then set for the affected families, the feature
> flags are unnecessarily modified. On systems where the F00F bug is
> actually present, clearing the flag first triggers the warning.
> 

It might be useful to explicitly say that these order of events work
fine on the BSP but only cause problems on APs.

> Fix this by not clearing the flag first. Clearing it first is relict from
> commit e2604b49e8a8 ("x86, cpu: Convert F00F bug detection").
> 

Yeah, I couldn't find any historical reason to clear the bug first and
then set it. Not a strong preference but shouldn't above be set the
"Fixes" commit instead of below?

> Fixes: ee8962082a44 ("x86/alternatives: Catch late X86_FEATURE modifiers")
> Signed-off-by: Richard Weinberger <richard@nod.at>
> ---
>  arch/x86/kernel/cpu/intel.c | 1 -
>  1 file changed, 1 deletion(-)
> 

The code change looks good to me.
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>

> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 646ff33c4651..73f6bf182333 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -394,7 +394,6 @@ static void intel_workarounds(struct cpuinfo_x86 *c)
>  	 * system. Announce that the fault handler will be checking 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;
>  

Re: [PATCH 2/2] [RFC] x86: Remove clear_cpu_bug()

[Sohil Mehta]

On 2/16/2026 2:43 AM, Richard Weinberger wrote:
> F00F was the only user of clear_cpu_bug().
> With no users left, remove this helper.
> 

Yeah, this function isn't used commonly. The 2nd last user was removed
~9 years back.

> Signed-off-by: Richard Weinberger <richard@nod.at>
> ---
>  arch/x86/include/asm/cpufeature.h | 1 -
>  1 file changed, 1 deletion(-)
> 

Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>

> 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))