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From: Ryan Roberts <ryan.roberts@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Joey Gouly <joey.gouly@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	David Hildenbrand <david@redhat.com>,
	Peter Xu <peterx@redhat.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Shivansh Vij <shivanshvij@outlook.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: [PATCH v3 1/3] arm64/mm: Refactor PMD_PRESENT_INVALID and PTE_PROT_NONE bits
Date: Wed,  1 May 2024 15:54:17 +0100
Message-Id: <20240501145419.1390363-2-ryan.roberts@arm.com>
In-Reply-To: <20240501145419.1390363-1-ryan.roberts@arm.com>
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Currently the PMD_PRESENT_INVALID and PTE_PROT_NONE functionality
explicitly occupy 2 bits in the PTE when PTE_VALID/PMD_SECT_VALID is
clear. This has 2 significant consequences:

  - PTE_PROT_NONE consumes a precious SW PTE bit that could be used for
    other things.
  - The swap pte layout must reserve those same 2 bits and ensure they
    are both always zero for a swap pte. It would be nice to reclaim at
    least one of those bits.

Note that while PMD_PRESENT_INVALID technically only applies to pmds,
the swap pte layout is common to ptes and pmds so we are currently
effectively reserving that bit at both levels.

Let's replace PMD_PRESENT_INVALID with a more generic
PTE_PRESENT_INVALID bit, which occupies the same position (bit 59) but
applies uniformly to page/block descriptors at any level. This bit is
only interpreted when PTE_VALID is clear. If it is set, then the pte is
still considered present; pte_present() returns true and all the fields
in the pte follow the HW interpretation (e.g. SW can safely call
pte_pfn(), etc). But crucially, the HW treats the pte as invalid and
will fault if it hits.

With this in place, we can remove PTE_PROT_NONE entirely and instead
represent PROT_NONE as a present but invalid pte (PTE_VALID=0,
PTE_INVALID=1) with PTE_USER=0 and PTE_UXN=1. This is a unique
combination that is not used anywhere else.

The net result is a clearer, simpler, more generic encoding scheme that
applies uniformly to all levels. Additionally we free up a PTE SW bit
and a swap pte bit (bit 58 in both cases).

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 arch/arm64/include/asm/pgtable-prot.h | 11 ++----
 arch/arm64/include/asm/pgtable.h      | 50 +++++++++++++++------------
 2 files changed, 30 insertions(+), 31 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index dd9ee67d1d87..cd8c06f5fb02 100644
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -18,14 +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_PROT_NONE		(_AT(pteval_t, 1) << 58) /* only when !PTE_VALID */
-
-/*
- * This bit indicates that the entry is present i.e. pmd_page()
- * still points to a valid huge page in memory even if the pmd
- * has been invalidated.
- */
-#define PMD_PRESENT_INVALID	(_AT(pteval_t, 1) << 59) /* only when !PMD_SECT_VALID */
+#define PTE_PRESENT_INVALID	(_AT(pteval_t, 1) << 59) /* 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)
@@ -103,7 +96,7 @@ static inline bool __pure lpa2_is_enabled(void)
 		__val;							\
 	 })
 
-#define PAGE_NONE		__pgprot(((_PAGE_DEFAULT) & ~PTE_VALID) | PTE_PROT_NONE | PTE_RDONLY | PTE_NG | PTE_PXN | PTE_UXN)
+#define PAGE_NONE		__pgprot(((_PAGE_DEFAULT) & ~PTE_VALID) | PTE_PRESENT_INVALID | PTE_RDONLY | PTE_NG | PTE_PXN | PTE_UXN)
 /* shared+writable pages are clean by default, hence PTE_RDONLY|PTE_WRITE */
 #define PAGE_SHARED		__pgprot(_PAGE_SHARED)
 #define PAGE_SHARED_EXEC	__pgprot(_PAGE_SHARED_EXEC)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index afdd56d26ad7..c0f4471423db 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -105,7 +105,7 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
 /*
  * The following only work if pte_present(). Undefined behaviour otherwise.
  */
-#define pte_present(pte)	(!!(pte_val(pte) & (PTE_VALID | PTE_PROT_NONE)))
+#define pte_present(pte)	(pte_valid(pte) || pte_present_invalid(pte))
 #define pte_young(pte)		(!!(pte_val(pte) & PTE_AF))
 #define pte_special(pte)	(!!(pte_val(pte) & PTE_SPECIAL))
 #define pte_write(pte)		(!!(pte_val(pte) & PTE_WRITE))
@@ -132,6 +132,8 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
 #define pte_dirty(pte)		(pte_sw_dirty(pte) || pte_hw_dirty(pte))
 
 #define pte_valid(pte)		(!!(pte_val(pte) & PTE_VALID))
+#define pte_present_invalid(pte) \
+	((pte_val(pte) & (PTE_VALID | PTE_PRESENT_INVALID)) == PTE_PRESENT_INVALID)
 /*
  * Execute-only user mappings do not have the PTE_USER bit set. All valid
  * kernel mappings have the PTE_UXN bit set.
@@ -261,6 +263,13 @@ static inline pte_t pte_mkpresent(pte_t pte)
 	return set_pte_bit(pte, __pgprot(PTE_VALID));
 }
 
+static inline pte_t pte_mkinvalid(pte_t pte)
+{
+	pte = set_pte_bit(pte, __pgprot(PTE_PRESENT_INVALID));
+	pte = clear_pte_bit(pte, __pgprot(PTE_VALID));
+	return pte;
+}
+
 static inline pmd_t pmd_mkcont(pmd_t pmd)
 {
 	return __pmd(pmd_val(pmd) | PMD_SECT_CONT);
@@ -469,7 +478,16 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
  */
 static inline int pte_protnone(pte_t pte)
 {
-	return (pte_val(pte) & (PTE_VALID | PTE_PROT_NONE)) == PTE_PROT_NONE;
+	/*
+	 * pte_present_invalid() tells us that the pte is invalid from HW
+	 * perspective but present from SW perspective, so the fields are to be
+	 * interpretted as per the HW layout. The second 2 checks are the unique
+	 * encoding that we use for PROT_NONE. It is insufficient to only use
+	 * the first check because we share the same encoding scheme with pmds
+	 * which support pmd_mkinvalid(), so can be present-invalid without
+	 * being PROT_NONE.
+	 */
+	return pte_present_invalid(pte) && !pte_user(pte) && !pte_user_exec(pte);
 }
 
 static inline int pmd_protnone(pmd_t pmd)
@@ -478,12 +496,7 @@ static inline int pmd_protnone(pmd_t pmd)
 }
 #endif
 
-#define pmd_present_invalid(pmd)     (!!(pmd_val(pmd) & PMD_PRESENT_INVALID))
-
-static inline int pmd_present(pmd_t pmd)
-{
-	return pte_present(pmd_pte(pmd)) || pmd_present_invalid(pmd);
-}
+#define pmd_present(pmd)	pte_present(pmd_pte(pmd))
 
 /*
  * THP definitions.
@@ -508,14 +521,7 @@ static inline int pmd_trans_huge(pmd_t pmd)
 #define pmd_mkclean(pmd)	pte_pmd(pte_mkclean(pmd_pte(pmd)))
 #define pmd_mkdirty(pmd)	pte_pmd(pte_mkdirty(pmd_pte(pmd)))
 #define pmd_mkyoung(pmd)	pte_pmd(pte_mkyoung(pmd_pte(pmd)))
-
-static inline pmd_t pmd_mkinvalid(pmd_t pmd)
-{
-	pmd = set_pmd_bit(pmd, __pgprot(PMD_PRESENT_INVALID));
-	pmd = clear_pmd_bit(pmd, __pgprot(PMD_SECT_VALID));
-
-	return pmd;
-}
+#define pmd_mkinvalid(pmd)	pte_pmd(pte_mkinvalid(pmd_pte(pmd)))
 
 #define pmd_thp_or_huge(pmd)	(pmd_huge(pmd) || pmd_trans_huge(pmd))
 
@@ -1027,8 +1033,8 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 	 * in MAIR_EL1. The mask below has to include PTE_ATTRINDX_MASK.
 	 */
 	const pteval_t mask = PTE_USER | PTE_PXN | PTE_UXN | PTE_RDONLY |
-			      PTE_PROT_NONE | PTE_VALID | PTE_WRITE | PTE_GP |
-			      PTE_ATTRINDX_MASK;
+			      PTE_PRESENT_INVALID | PTE_VALID | PTE_WRITE |
+			      PTE_GP | PTE_ATTRINDX_MASK;
 	/* preserve the hardware dirty information */
 	if (pte_hw_dirty(pte))
 		pte = set_pte_bit(pte, __pgprot(PTE_DIRTY));
@@ -1076,17 +1082,17 @@ static inline int pgd_devmap(pgd_t pgd)
 #ifdef CONFIG_PAGE_TABLE_CHECK
 static inline bool pte_user_accessible_page(pte_t pte)
 {
-	return pte_present(pte) && (pte_user(pte) || pte_user_exec(pte));
+	return pte_valid(pte) && (pte_user(pte) || pte_user_exec(pte));
 }
 
 static inline bool pmd_user_accessible_page(pmd_t pmd)
 {
-	return pmd_leaf(pmd) && !pmd_present_invalid(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
+	return pmd_valid(pmd) && !pmd_table(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
 }
 
 static inline bool pud_user_accessible_page(pud_t pud)
 {
-	return pud_leaf(pud) && (pud_user(pud) || pud_user_exec(pud));
+	return pud_valid(pud) && !pud_table(pud) && (pud_user(pud) || pud_user_exec(pud));
 }
 #endif
 
@@ -1250,7 +1256,7 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
  *	bits 2:		remember PG_anon_exclusive
  *	bits 3-7:	swap type
  *	bits 8-57:	swap offset
- *	bit  58:	PTE_PROT_NONE (must be zero)
+ *	bit  59:	PTE_PRESENT_INVALID (must be zero)
  */
 #define __SWP_TYPE_SHIFT	3
 #define __SWP_TYPE_BITS		5
-- 
2.25.1