Re: [PATCH v6 1/3] x86/process: Shorten the LAM tag width

[Sohil Mehta <sohil.mehta@intel.com>]

On 4/10/2026 2:55 AM, Maciej Wieczor-Retman wrote:
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 08e72f429870..d6f8e71156cd 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -797,7 +797,10 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
>  
>  #ifdef CONFIG_ADDRESS_MASKING
>  
> -#define LAM_U57_BITS 6
> +#define LAM_TAG_BITS	4

This makes sense as the tag width would be fixed at 4.

> +#define LAM_LS_BIT	57
> +#define LAM_MS_BIT	(LAM_LS_BIT + LAM_TAG_BITS - 1) /* 60 */
> +#define LAM_UNTAG_MASK	~GENMASK(LAM_MS_BIT, LAM_LS_BIT)
>  

This seems overkill. What we essentially need is 4 bits starting from
bit 57. GENMASK is the right macro for that, but it doesn't need these
extra argument macros and tail comments.

I think using direct numbers with a simple comment on top would be
better. How about something like?

/* Must use the 4 lower bits from the LAM_U57 tag bits 62:57 */
#define LAM_UNTAG_MASK	~GENMASK(60, 57)

You can probably add some explanation in the commit log about why only
the lower bits can be used.

>  static void enable_lam_func(void *__mm)
>  {
> @@ -814,7 +817,7 @@ static void enable_lam_func(void *__mm)
>  static void mm_enable_lam(struct mm_struct *mm)
>  {
>  	mm->context.lam_cr3_mask = X86_CR3_LAM_U57;
> -	mm->context.untag_mask =  ~GENMASK(62, 57);
> +	mm->context.untag_mask = LAM_UNTAG_MASK;
>  


Other than that,

Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>