Re: [PATCHv8 00/11] Linear Address Masking enabling

[Peter Zijlstra]

On Mon, Sep 05, 2022 at 06:35:17PM +0300, Kirill A. Shutemov wrote:
> What about something like this?
> 
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 803241dfc473..868d2730884b 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -30,8 +30,10 @@ static inline bool pagefault_disabled(void);
>   */
>  #define untagged_addr(mm, addr)	({					\
>  	u64 __addr = (__force u64)(addr);				\
> -	s64 sign = (s64)__addr >> 63;					\
> -	__addr &= (mm)->context.untag_mask | sign;			\
> +	if (static_cpu_has(X86_FEATURE_LAM)) {				\
> +		s64 sign = (s64)__addr >> 63;				\
> +		__addr &= (mm)->context.untag_mask | sign;		\
> +	}								\
>  	(__force __typeof__(addr))__addr;				\
>  })

Well, if you go throught the trouble of adding it, might as well use a
regular static branch and only enable it once there's an actual user,
no?