Re: [PATCH v3 2/2] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

Re: [PATCH v3 2/2] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

[kernel test robot]

Hello,

kernel test robot noticed "WARNING:at_arch/x86/kernel/fpu/init.c:#fpu__init_cpu" on:

commit: b88c4665c7f43e1898f695642fd159c6c542e49b ("[PATCH v3 2/2] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining")
url: https://github.com/intel-lab-lkp/linux/commits/Eric-Biggers/x86-fpu-Replace-in_kernel_fpu-with-kernel_fpu_allowed/20250520-233322
patch link: https://lore.kernel.org/all/20250520152938.21881-3-ebiggers@kernel.org/
patch subject: [PATCH v3 2/2] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

in testcase: boot

config: i386-randconfig-003-20250522
compiler: clang-20
test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G

(please refer to attached dmesg/kmsg for entire log/backtrace)


+------------------------------------------------------+------------+------------+
|                                                      | 5454801de7 | b88c4665c7 |
+------------------------------------------------------+------------+------------+
| WARNING:at_arch/x86/kernel/fpu/init.c:#fpu__init_cpu | 0          | 12         |
| EIP:fpu__init_cpu                                    | 0          | 12         |
+------------------------------------------------------+------------+------------+


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 <oliver.sang@intel.com>
| Closes: https://lore.kernel.org/oe-lkp/202505280957.3efe5bf5-lkp@intel.com


[    0.324937][    T0] ------------[ cut here ]------------
[ 0.325455][ T0] WARNING: CPU: 0 PID: 0 at arch/x86/kernel/fpu/init.c:56 fpu__init_cpu (arch/x86/kernel/fpu/init.c:56 (discriminator 15)) 
[    0.326299][    T0] Modules linked in:
[    0.326690][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Tainted: G                T   6.15.0-rc7-00706-gb88c4665c7f4 #1 PREEMPTLAZY  c76c09082e833299bbcc71b75dc4abf37758b94a
[    0.328079][    T0] Tainted: [T]=RANDSTRUCT
[    0.328493][    T0] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
[ 0.329028][ T0] EIP: fpu__init_cpu (arch/x86/kernel/fpu/init.c:56 (discriminator 15)) 
[ 0.329483][ T0] Code: 75 00 00 db e3 e8 64 28 00 00 a0 10 76 8a 83 84 c0 75 17 c6 05 10 76 8a 83 01 83 c4 08 5e 5f 5d 31 c0 31 d2 2e e9 58 cc 8c 01 <0f> 0b eb e5 e8 b3 0b 8c 01 f7 c7 00 02 00 00 75 bc eb bb 90 55 89
All code
========
   0:	75 00                	jne    0x2
   2:	00 db                	add    %bl,%bl
   4:	e3 e8                	jrcxz  0xffffffffffffffee
   6:	64 28 00             	sub    %al,%fs:(%rax)
   9:	00 a0 10 76 8a 83    	add    %ah,-0x7c7589f0(%rax)
   f:	84 c0                	test   %al,%al
  11:	75 17                	jne    0x2a
  13:	c6 05 10 76 8a 83 01 	movb   $0x1,-0x7c7589f0(%rip)        # 0xffffffff838a762a
  1a:	83 c4 08             	add    $0x8,%esp
  1d:	5e                   	pop    %rsi
  1e:	5f                   	pop    %rdi
  1f:	5d                   	pop    %rbp
  20:	31 c0                	xor    %eax,%eax
  22:	31 d2                	xor    %edx,%edx
  24:	2e e9 58 cc 8c 01    	cs jmp 0x18ccc82
  2a:*	0f 0b                	ud2		<-- trapping instruction
  2c:	eb e5                	jmp    0x13
  2e:	e8 b3 0b 8c 01       	call   0x18c0be6
  33:	f7 c7 00 02 00 00    	test   $0x200,%edi
  39:	75 bc                	jne    0xfffffffffffffff7
  3b:	eb bb                	jmp    0xfffffffffffffff8
  3d:	90                   	nop
  3e:	55                   	push   %rbp
  3f:	89                   	.byte 0x89

Code starting with the faulting instruction
===========================================
   0:	0f 0b                	ud2
   2:	eb e5                	jmp    0xffffffffffffffe9
   4:	e8 b3 0b 8c 01       	call   0x18c0bbc
   9:	f7 c7 00 02 00 00    	test   $0x200,%edi
   f:	75 bc                	jne    0xffffffffffffffcd
  11:	eb bb                	jmp    0xffffffffffffffce
  13:	90                   	nop
  14:	55                   	push   %rbp
  15:	89                   	.byte 0x89
[    0.331175][    T0] EAX: 00000001 EBX: 00020801 ECX: 00000000 EDX: 00000000
[    0.332290][    T0] ESI: 00000600 EDI: 00200206 EBP: 83841f78 ESP: 83841f68
[    0.332937][    T0] DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068 EFLAGS: 00210202
[    0.333655][    T0] CR0: 80050033 CR2: ffbff000 CR3: 0413a000 CR4: 000406b0
[    0.334516][    T0] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
[    0.335460][    T0] DR6: fffe0ff0 DR7: 00000400
[    0.336093][    T0] Call Trace:
[ 0.336599][ T0] arch_cpu_finalize_init (arch/x86/kernel/cpu/common.c:2536) 
[ 0.337340][ T0] start_kernel (init/main.c:1067) 
[ 0.338019][ T0] i386_start_kernel (arch/x86/kernel/head32.c:79 (discriminator 1)) 
[ 0.338725][ T0] startup_32_smp (arch/x86/kernel/head_32.S:290) 
[    0.339030][    T0] irq event stamp: 2667
[ 0.339652][ T0] hardirqs last enabled at (2677): __console_unlock (arch/x86/include/asm/irqflags.h:19 arch/x86/include/asm/irqflags.h:109 arch/x86/include/asm/irqflags.h:151 kernel/printk/printk.c:344 kernel/printk/printk.c:2885) 
[ 0.340902][ T0] hardirqs last disabled at (2686): __console_unlock (kernel/printk/printk.c:342 (discriminator 9)) 
[ 0.342501][ T0] softirqs last enabled at (0): 0x0 
[ 0.345464][ T0] softirqs last disabled at (0): 0x0 
[    0.346433][    T0] ---[ end trace 0000000000000000 ]---


The kernel config and materials to reproduce are available at:
https://download.01.org/0day-ci/archive/20250528/202505280957.3efe5bf5-lkp@intel.com



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

Re: [PATCH v3 2/2] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

[Eric Biggers]

On Wed, May 28, 2025 at 10:04:39AM +0800, kernel test robot wrote:
> 
> Hello,
> 
> kernel test robot noticed "WARNING:at_arch/x86/kernel/fpu/init.c:#fpu__init_cpu" on:
> 
> commit: b88c4665c7f43e1898f695642fd159c6c542e49b ("[PATCH v3 2/2] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining")
> url: https://github.com/intel-lab-lkp/linux/commits/Eric-Biggers/x86-fpu-Replace-in_kernel_fpu-with-kernel_fpu_allowed/20250520-233322
> patch link: https://lore.kernel.org/all/20250520152938.21881-3-ebiggers@kernel.org/
> patch subject: [PATCH v3 2/2] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

Right, this is because fpu__init_cpu() is actually called twice on the boot CPU.
So the WARN_ON_FPU I added in v2 of this patch trips.  Fortunately, the version
that was applied was v1, and it does not have the problematic WARN_ON_FPU.

I wonder if fpu__init_cpu() really should be called twice.  The flow is:

    arch_cpu_finalize_init()
        fpu__init_system()
            fpu__init_system_early_generic()
            fpu__init_cpu()
            fpu__init_system_generic();
            fpu__init_system_xstate_size_legacy();
            fpu__init_system_xstate(fpu_kernel_cfg.max_size);
            fpu__init_task_struct_size();
        fpu__init_cpu()

- Eric