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arm64/mm: Move PTE_PRESENT_INVALID to overlay PTE_NG

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PTE_PRESENT_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_PRESENT_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_PRESENT_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_PRESENT_INVALID to a low bit we can lay the swap pte out so that the
offset field could grow to 52 bits in future.

So let's move PTE_PRESENT_INVALID to overlay PTE_NG (bit 11).

There is no need to persist NG for a present-invalid entry; it is always
set for user mappings and is not used by SW to derive any state from the
pte. PTE_NS was considered instead of PTE_NG, but it is RES0 for
non-secure SW, so there is a chance that future architecture may
allocate the bit and we may therefore need to persist that bit for
present-invalid ptes.

These are both marginal benefits, but make things a bit tidier in my
opinion.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20240503144604.151095-4-ryan.roberts@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Ryan Roberts 2024-05-03 15:46:01 +01:00 committed by Will Deacon
parent f0f5863a0f
commit 55564814a8
2 changed files with 7 additions and 7 deletions
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2
arch/arm64/include/asm/pgtable-prot.h
View file

@ -24,7 +24,7 @@
* interpreted according to the HW layout by SW but any attempted HW access to
* the address will result in a fault. pte_present() returns true.
*/
#define PTE_PRESENT_INVALID (_AT(pteval_t, 1) << 59) /* only when !PTE_VALID */
#define PTE_PRESENT_INVALID (PTE_NG) /* 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)

12
arch/arm64/include/asm/pgtable.h
View file

@ -1261,15 +1261,15 @@ 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_PRESENT_INVALID (must be zero)
* bits 6-10: swap type
* bit 11: PTE_PRESENT_INVALID (must be zero)
* bits 12-61: 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)
#define __SWP_OFFSET_SHIFT (__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)
#define __SWP_OFFSET_SHIFT 12
#define __SWP_OFFSET_BITS 50
#define __SWP_OFFSET_MASK ((1UL << __SWP_OFFSET_BITS) - 1)
#define __swp_type(x) (((x).val >> __SWP_TYPE_SHIFT) & __SWP_TYPE_MASK)