[PATCH v2 1/3] arm64/mm: Refactor PMD_PRESENT_INVALID and PTE_PROT_NONE bits

Ryan Roberts
2024-04-29T14:02:21Z

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_INVALID bit,
which occupies the same position (bit 59) but applies uniformly to
page/block descriptors at any level. This bit is only interpretted 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 a
swap pte bit (bit 58 in both cases).

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
arch/arm64/include/asm/pgtable-prot.h | 11 ++------
arch/arm64/include/asm/pgtable.h | 38 ++++++++++++---------------
2 files changed, 19 insertions(+), 30 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index dd9ee67d1d87..de62e6881154 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_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_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..8dd4637d6b56 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_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 +132,7 @@ 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_invalid(pte) ((pte_val(pte) & (PTE_VALID | PTE_INVALID)) == PTE_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 +262,13 @@ 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_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 +477,7 @@ 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;
+ return pte_invalid(pte) && !pte_user(pte) && !pte_user_exec(pte);
}

static inline int pmd_protnone(pmd_t pmd)
@@ -478,12 +486,7 @@ 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 +511,7 @@ 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))

@@ -1027,7 +1023,7 @@ 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_INVALID | PTE_VALID | PTE_WRITE | PTE_GP |
PTE_ATTRINDX_MASK;
/* preserve the hardware dirty information */
if (pte_hw_dirty(pte))
@@ -1076,17 +1072,17 @@ 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 +1246,7 @@ 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_INVALID (must be zero)
*/
#define __SWP_TYPE_SHIFT 3
#define __SWP_TYPE_BITS 5
--
2.25.1