Re: [PATCH v8 09/33] x86/cpu: add X86_CR4_FRED macro

[Thomas Gleixner]

On Mon, Apr 10 2023 at 01:14, Xin Li wrote:

> From: "H. Peter Anvin (Intel)" <hpa@xxxxxxxxx>
>
> Add X86_CR4_FRED macro for the FRED bit in %cr4. This bit should be a

s/should/must/ no?

> +/*
> + * These bits should not change their value after CPU init is finished.
> + * The explicit cast to unsigned long suppresses a warning on i386 for
> + * x86-64 only feature bits >= 32.
> + */
>  static const unsigned long cr4_pinned_mask =
> -	X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
> -	X86_CR4_FSGSBASE | X86_CR4_CET;
> +	(unsigned long)

That type cast is required because:

+#define X86_CR4_FRED		_BITULL(X86_CR4_FRED_BIT)

Bah. Fred is 64 bit only. So why defining this as 1ULL << 32
unconditionally and stripping the bit off on 32bit via the type cast?

#ifdef CONFIG_X86_64
#define X86_CR4_FRED		_BITUL(X86_CR4_FRED_BIT)
#else
#define X86_CR4_FRED		0
#endif

would be too obvious, right?

> +	(X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
> +	 X86_CR4_FSGSBASE | X86_CR4_CET | X86_CR4_FRED);

Thanks,

        tglx