Re: [PATCH 09/10] x86/fpu: Remove unnecessary CPUID level check

[Chang S. Bae]

On 12/13/2024 12:50 PM, Dave Hansen wrote:
> diff -puN arch/x86/kernel/fpu/xstate.c~xsave-leaf-checks-2 arch/x86/kernel/fpu/xstate.c
> --- a/arch/x86/kernel/fpu/xstate.c~xsave-leaf-checks-2	2024-12-13 12:45:36.903226562 -0800
> +++ b/arch/x86/kernel/fpu/xstate.c	2024-12-13 12:45:36.907226732 -0800
> @@ -764,11 +764,6 @@ void __init fpu__init_system_xstate(unsi
>   		return;
>   	}
>   
> -	if (boot_cpu_data.cpuid_level < XSTATE_CPUID) {
> -		WARN_ON_FPU(1);
> -		return;
> -	}
> -

I think this change effectively reverts commit ee813d53a8e9 ("x86, 
xsave: Check cpuid level for XSTATE_CPUID (0x0d)").

At the time of its introduction, commit b38b06659055 ("x86: filter CPU 
features dependent on unavailable CPUID levels") was already there. The 
cpu_has_xsave flag (now boot_cpu_has(X86_FEATURE_XSAVE)) should be false 
for CPUs with a lower CPUID level in this:

void __cpuinit xsave_init(void)
{
    if (!cpu_has_xsave)
        return;
    ...
       xstate_enable_boot_cpu();
}

Then, the sanity check introduced in this code path appears redundant:

xsave_init()
-> xstate_enable_boot_cpu()
    -> setup_xstate_init()
        -> setup_xstate_features()

Reviewed-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>

Thanks,
Chang