Re: [PATCH v2 1/4] x86/fpu: Add kernel_fpu_begin_mask() to selectively initialize state

[Sean Christopherson]

On Tue, Jan 19, 2021, Andy Lutomirski wrote:
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index eb86a2b831b1..d4a71596c41e 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -121,7 +121,7 @@ int copy_fpregs_to_fpstate(struct fpu *fpu)
>  }
>  EXPORT_SYMBOL(copy_fpregs_to_fpstate);
>  
> -void kernel_fpu_begin(void)
> +void kernel_fpu_begin_mask(unsigned int kfpu_mask)
>  {
>       preempt_disable();
>  
> @@ -141,13 +141,18 @@ void kernel_fpu_begin(void)
>       }
>       __cpu_invalidate_fpregs_state();
>  
> -     if (boot_cpu_has(X86_FEATURE_XMM))
> -             ldmxcsr(MXCSR_DEFAULT);
> +     /* Put sane initial values into the control registers. */
> +     if (likely(kfpu_mask & KFPU_MXCSR)) {
> +             if (boot_cpu_has(X86_FEATURE_XMM))
> +                     ldmxcsr(MXCSR_DEFAULT);
> +     }
>  
> -     if (boot_cpu_has(X86_FEATURE_FPU))
> -             asm volatile ("fninit");
> +     if (unlikely(kfpu_mask & KFPU_387)) {
> +             if (boot_cpu_has(X86_FEATURE_FPU))
> +                     asm volatile ("fninit");
> +     }

Why not combine these into a single if statement?  Easier on the eyes (IMO), and
would generate a smaller diff.

        if (likely(kfpu_mask & KFPU_MXCSR) && boot_cpu_has(X86_FEATURE_XMM))
                ldmxcsr(MXCSR_DEFAULT);

        if (unlikely(kfpu_mask & KFPU_387) && boot_cpu_has(X86_FEATURE_FPU))
                asm volatile ("fninit");

>  }
> -EXPORT_SYMBOL_GPL(kernel_fpu_begin);
> +EXPORT_SYMBOL_GPL(kernel_fpu_begin_mask);
>  
>  void kernel_fpu_end(void)
>  {
> -- 
> 2.29.2
>