arm64/mm: Remove PTE_PROT_NONE bit

Currently the PTE_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.

But PTE_PRESENT_INVALID, which since the previous patch, applies
uniformly to page/block descriptors at any level when PTE_VALID is
clear, can already give us most of what PTE_PROT_NONE requires: 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.

So let's remove PTE_PROT_NONE entirely and instead represent PROT_NONE
as a present but invalid pte (PTE_VALID=0, PTE_PRESENT_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>
Link: https://lore.kernel.org/r/20240503144604.151095-3-ryan.roberts@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index cdbf51eef7a6e1fe5ba14f0881d0ffd1ebbae6ad..81f07b44f7b8e82e0810dd5f4c505649e148a2f6 100644 (file)
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -18,7 +18,6 @@
 #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 */
 
 /*
  * PTE_PRESENT_INVALID=1 & PTE_VALID=0 indicates that the pte's fields should be
@@ -103,7 +102,7 @@ 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 a3e417f388e9c91ce095f8f4bfee075510e77d8f..300abbc0e67a7a7a07f5440354bc495b4caf7985 100644 (file)
--- 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))
@@ -483,7 +483,16 @@ 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)
@@ -492,12 +501,7 @@ static inline int pmd_protnone(pmd_t pmd)
 }
 #endif
 
-#define pmd_present_invalid(pmd)       pte_present_invalid(pmd_pte(pmd))
-
-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.
@@ -1036,8 +1040,8 @@ 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));
@@ -1085,17 +1089,17 @@ 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
 
@@ -1259,7 +1263,6 @@ 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