linux-kernel - [PATCH v2 2/3] arm64/mm: Move PTE_INVALID to overlay PTE

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Date: Mon, 29 Apr 2024 15:02:06 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Joey Gouly <joey.gouly@....com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	David Hildenbrand <david@...hat.com>,
	Peter Xu <peterx@...hat.com>,
	Mike Rapoport <rppt@...ux.ibm.com>,
	Shivansh Vij <shivanshvij@...look.com>
Cc: Ryan Roberts <ryan.roberts@....com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/3] arm64/mm: Move PTE_INVALID to overlay PTE_NS

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@....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