linux-kernel - Re: [PATCH v2 1/4] x86/fpu: Add kernel_fpu_begin

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Date:   Tue, 19 Jan 2021 11:34:57 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Krzysztof Mazur <krzysiek@...lesie.net>,
        Krzysztof Olędzki <ole@....pl>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 1/4] x86/fpu: Add kernel_fpu_begin_mask() to
 selectively initialize state

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
>