Re: [PATCH] x86/fpu: set X86_FEATURE_OSXSAVE feature after enabling OSXSAVE in CR4

[Thomas Gleixner]

On Wed, Aug 23 2023 at 14:57, Feng Tang wrote:
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 0bab497c9436..8ebea0d522d2 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -173,6 +173,9 @@ void fpu__init_cpu_xstate(void)
>  
>  	cr4_set_bits(X86_CR4_OSXSAVE);
>  
> +	if (!boot_cpu_has(X86_FEATURE_OSXSAVE))
> +		setup_force_cpu_cap(X86_FEATURE_OSXSAVE);

This is wrong in several aspects:

     1) You force the feature bit _before_ XSAVE is completely
        initialized. fpu__init_system_xstate() has error paths which
        disable XSAVE.

     2) This conditional should have been a red flag for you simply
        because fpu__init_cpu_xstate() is invoked on all CPUs not only
        on the BSP.

I fixed it up and added a proper comment explaining it.