arm64/mm: Move PTE_PRESENT_INVALID to overlay PTE_NG

PTE_PRESENT_INVALID was previously occupying bit 59, which when a PTE is
valid can either be IGNORED, PBHA[0] or AttrIndex[3], depending on the
HW configuration. In practice this is currently not a problem because
PTE_PRESENT_INVALID can only be 1 when PTE_VALID=0 and upstream Linux
always requires the bit set to 0 for a valid pte.

However, if in future Linux wants to use the field (e.g. AttrIndex[3])
then we could end up with confusion when PTE_PRESENT_INVALID comes along
and corrupts the field - we would ideally want to preserve it even for
an invalid (but present) pte.

The other problem with bit 59 is that it prevents the offset field of a
swap entry within a swap pte from growing beyond 51 bits. By moving
PTE_PRESENT_INVALID to a low bit we can lay the swap pte out so that the
offset field could grow to 52 bits in future.

So let's move PTE_PRESENT_INVALID to overlay PTE_NG (bit 11).

There is no need to persist NG for a present-invalid entry; it is always
set for user mappings and is not used by SW to derive any state from the
pte. PTE_NS was considered instead of PTE_NG, but it is RES0 for
non-secure SW, so there is a chance that future architecture may
allocate the bit and we may therefore need to persist that bit for
present-invalid ptes.

These are both marginal benefits, but make things a bit tidier in my
opinion.
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20240503144604.151095-4-ryan.roberts@arm.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/pgtable-prot.h

@@ -24,7 +24,7 @@
  * interpreted according to the HW layout by SW but any attempted HW access to
  * the address will result in a fault. pte_present() returns true.
  */
-#define PTE_PRESENT_INVALID	(_AT(pteval_t, 1) << 59) /* only when !PTE_VALID */
+#define PTE_PRESENT_INVALID	(PTE_NG)		 /* only when !PTE_VALID */
 
 #define _PROT_DEFAULT		(PTE_TYPE_PAGE | PTE_AF | PTE_SHARED)
 #define _PROT_SECT_DEFAULT	(PMD_TYPE_SECT | PMD_SECT_AF | PMD_SECT_S)

arch/arm64/include/asm/pgtable.h

@@ -1261,15 +1261,15 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
  * Encode and decode a swap entry:
  *	bits 0-1:	present (must be zero)
  *	bits 2:		remember PG_anon_exclusive
- *	bits 3-7:	swap type
- *	bits 8-57:	swap offset
- *	bit  59:	PTE_PRESENT_INVALID (must be zero)
+ *	bits 6-10:	swap type
+ *	bit  11:	PTE_PRESENT_INVALID (must be zero)
+ *	bits 12-61:	swap offset
  */
-#define __SWP_TYPE_SHIFT	3
+#define __SWP_TYPE_SHIFT	6
 #define __SWP_TYPE_BITS		5
-#define __SWP_OFFSET_BITS	50
 #define __SWP_TYPE_MASK		((1 << __SWP_TYPE_BITS) - 1)
-#define __SWP_OFFSET_SHIFT	(__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)
+#define __SWP_OFFSET_SHIFT	12
+#define __SWP_OFFSET_BITS	50
 #define __SWP_OFFSET_MASK	((1UL << __SWP_OFFSET_BITS) - 1)
 
 #define __swp_type(x)		(((x).val >> __SWP_TYPE_SHIFT) & __SWP_TYPE_MASK)