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arm64/mm: Remove PTE_PROT_NONE bit

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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>
This commit is contained in:
Ryan Roberts 2024-05-03 15:46:00 +01:00 committed by Will Deacon
parent b28c74e259
commit f0f5863a0f
2 changed files with 18 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
arch/arm64/include/asm/pgtable-prot.h
View File

@ -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)

31
arch/arm64/include/asm/pgtable.h
View File

@ -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