Re: [PATCH v14 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES supervisor states

[Cyrill Gorcunov]

On Mon, Oct 12, 2020 at 08:38:27AM -0700, Yu-cheng Yu wrote:
...
>  /*
>   * x86-64 Task Priority Register, CR8
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 038e19c0019e..705fd9b94e31 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -38,6 +38,9 @@ static const char *xfeature_names[] =
>  	"Processor Trace (unused)"	,
>  	"Protection Keys User registers",
>  	"unknown xstate feature"	,
> +	"Control-flow User registers"	,
> +	"Control-flow Kernel registers"	,
> +	"unknown xstate feature"	,
>  };
>  
>  static short xsave_cpuid_features[] __initdata = {
> @@ -51,6 +54,9 @@ static short xsave_cpuid_features[] __initdata = {
>  	X86_FEATURE_AVX512F,
>  	X86_FEATURE_INTEL_PT,
>  	X86_FEATURE_PKU,
> +	-1,		   /* Unused */
> +	X86_FEATURE_SHSTK, /* XFEATURE_CET_USER */
> +	X86_FEATURE_SHSTK, /* XFEATURE_CET_KERNEL */
>  };

Why do you need "-1" here in the array? The only 1:1 mapping is between
the names itselves and values, not indices of arrays so i don't understand
why we need this unused value. Sorry if it is a dumb questions and
been discussed already.