[PATCH v2 2/3] arm64/mm: Move PTE_INVALID to overlay PTE_NS

Ryan Roberts
2024-04-29T14:02:23Z

PTE_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_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_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_INVALID to a low bit we can lay the swap pte out so that the
offset field could grow to 53 bits in future.

So let's move PTE_INVALID to overlay PTE_NS (bit 5). PTE_NS is res0 for
SW outside of the secure state so Linux will never need to touch it.

These are both marginal benefits, but make things a bit tidier in my
opinion.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
arch/arm64/include/asm/pgtable-hwdef.h | 1 +
arch/arm64/include/asm/pgtable-prot.h | 2 +-
arch/arm64/include/asm/pgtable.h | 8 ++++----
3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index ef207a0d4f0d..7e1fea3a4328 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -160,6 +160,7 @@
#define PTE_TYPE_MASK (_AT(pteval_t, 3) << 0)
#define PTE_TYPE_PAGE (_AT(pteval_t, 3) << 0)
#define PTE_TABLE_BIT (_AT(pteval_t, 1) << 1)
+#define PTE_NS (_AT(pteval_t, 1) << 5) /* NS */
#define PTE_USER (_AT(pteval_t, 1) << 6) /* AP[1] */
#define PTE_RDONLY (_AT(pteval_t, 1) << 7) /* AP[2] */
#define PTE_SHARED (_AT(pteval_t, 3) << 8) /* SH[1:0], inner shareable */
diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index de62e6881154..ddf55895c9c2 100644
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -18,7 +18,7 @@
#define PTE_DIRTY (_AT(pteval_t, 1) << 55)
#define PTE_SPECIAL (_AT(pteval_t, 1) << 56)
#define PTE_DEVMAP (_AT(pteval_t, 1) << 57)
-#define PTE_INVALID (_AT(pteval_t, 1) << 59) /* only when !PTE_VALID */
+#define PTE_INVALID (PTE_NS) /* 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)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 8dd4637d6b56..d966d2ee1097 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1244,11 +1244,11 @@ 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_INVALID (must be zero)
+ * bit 5: PTE_INVALID (must be zero)
+ * bits 6-10: swap type
+ * bits 11-60: 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)
--
2.25.1