[PATCH V2] arm64/mm: Implement pte_po_index() for permission overlay index

[Anshuman Khandual <anshuman.khandual@arm.com>]

From: Ryan Roberts <ryan.roberts@arm.com>

Previously pte_access_permitted() used FIELD_GET() directly to retrieve
the permission overlay index from the pte. However, FIELD_GET() doesn't
work for 128 bit quanitites. Since we are about to add support for D128
pgtables, let's create a specific helper, pte_po_index() which can do
the required mask and shift regardless of the data type width.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: kvmarm@lists.linux.dev
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
This patch applies on v6.15-rc2

Changes in V2:

- Replaced FIELD_GET() in compute_s1_overlay_permissions() per Ryan

Changes in V1:

https://lore.kernel.org/linux-arm-kernel/20250410052021.1533180-1-anshuman.khandual@arm.com/

 arch/arm64/include/asm/pgtable-hwdef.h | 1 +
 arch/arm64/include/asm/pgtable-prot.h  | 2 ++
 arch/arm64/include/asm/pgtable.h       | 2 +-
 arch/arm64/kvm/at.c                    | 2 +-
 4 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index f3b77deedfa2..028a164924df 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -211,6 +211,7 @@
 #define PTE_PO_IDX_2	(_AT(pteval_t, 1) << 62)
 
 #define PTE_PO_IDX_MASK		GENMASK_ULL(62, 60)
+#define PTE_PO_IDX_SHIFT	60
 
 
 /*
diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index 7830d031742e..b53bc241e4e7 100644
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -136,6 +136,8 @@ static inline bool __pure lpa2_is_enabled(void)
 	((pte & BIT(PTE_PI_IDX_1)) >> (PTE_PI_IDX_1 - 1)) | \
 	((pte & BIT(PTE_PI_IDX_0)) >> (PTE_PI_IDX_0 - 0)))
 
+#define pte_po_index(pte)	((pte_val(pte) & PTE_PO_IDX_MASK) >> PTE_PO_IDX_SHIFT)
+
 /*
  * Page types used via Permission Indirection Extension (PIE). PIE uses
  * the USER, DBM, PXN and UXN bits to to generate an index which is used
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index d3b538be1500..41979c0e6c21 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -182,7 +182,7 @@ static inline bool por_el0_allows_pkey(u8 pkey, bool write, bool execute)
 	(((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER)) && (!(write) || pte_write(pte)))
 #define pte_access_permitted(pte, write) \
 	(pte_access_permitted_no_overlay(pte, write) && \
-	por_el0_allows_pkey(FIELD_GET(PTE_PO_IDX_MASK, pte_val(pte)), write, false))
+	por_el0_allows_pkey(pte_po_index(pte), write, false))
 #define pmd_access_permitted(pmd, write) \
 	(pte_access_permitted(pmd_pte(pmd), (write)))
 #define pud_access_permitted(pud, write) \
diff --git a/arch/arm64/kvm/at.c b/arch/arm64/kvm/at.c
index f74a66ce3064..66bee3f2d7c2 100644
--- a/arch/arm64/kvm/at.c
+++ b/arch/arm64/kvm/at.c
@@ -1073,7 +1073,7 @@ static void compute_s1_overlay_permissions(struct kvm_vcpu *vcpu,
 {
 	u8 idx, pov_perms, uov_perms;
 
-	idx = FIELD_GET(PTE_PO_IDX_MASK, wr->desc);
+	idx = pte_po_index(__pte(wr->desc));
 
 	switch (wi->regime) {
 	case TR_EL10:
-- 
2.25.1