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

[Eric Biggers <ebiggers@kernel.org>]

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