Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove tags before address check

[Peter Zijlstra]

On Fri, Jun 10, 2022 at 05:35:24PM +0300, Kirill A. Shutemov wrote:
> +#ifdef CONFIG_X86_64
> +/*
> + * Mask out tag bits from the address.
> + *
> + * Magic with the 'sign' allows to untag userspace pointer without any branches
> + * while leaving kernel addresses intact.
> + */
> +#define untagged_addr(mm, addr)	({					\
> +	u64 __addr = (__force u64)(addr);				\
> +	s64 sign = (s64)__addr >> 63;					\
> +	__addr ^= sign;							\
> +	__addr &= (mm)->context.untag_mask;				\
> +	__addr ^= sign;							\
> +	(__force __typeof__(addr))__addr;				\
> +})

Can't we make that mask a constant and *always* unmask U57 irrespective
of LAM being on?