[RFC PATCH v2 1/3] KVM: arm64: pgtables: Set DBM attribute on writable Stage-2 PTEs

[Leonardo Bras]

As a preparation to the introduction of HAFDBS-based features, set the DBM
bit (along with S2AP[1]) to all writable PTEs. Also unset DBM and S2AP[1]
when a PTE is made read-only again.

DBM bit works as following:
- If HAFDBS is disabled, DBM bit is ignored.
- If HAFDBS is enabled, and DBM bit is set, a write to non-writable page
  can promote it's PTE to writable (S2AP[1]=1) without taking a fault.

Since it's being set on writable and unset on read-only, it should cause no
functional change.

Signed-off-by: Leonardo Bras <leo.bras@xxxxxxx>
---
 arch/arm64/include/asm/kvm_pgtable.h | 3 +++
 arch/arm64/kvm/hyp/pgtable.c         | 5 +++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 41a8687938eb..37baa86d6fd8 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -91,14 +91,17 @@ typedef u64 kvm_pte_t;
 #define KVM_PTE_LEAF_ATTR_HI_S1_UXN	BIT(54)
 #define KVM_PTE_LEAF_ATTR_HI_S1_PXN	BIT(53)
 
 #define KVM_PTE_LEAF_ATTR_HI_S2_XN	GENMASK(54, 53)
 
+#define KVM_PTE_LEAF_ATTR_HI_S2_DBM	BIT(51)
+
 #define KVM_PTE_LEAF_ATTR_HI_S1_GP	BIT(50)
 
 #define KVM_PTE_LEAF_ATTR_S2_PERMS	(KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R | \
 					 KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W | \
+					 KVM_PTE_LEAF_ATTR_HI_S2_DBM | \
 					 KVM_PTE_LEAF_ATTR_HI_S2_XN)
 
 /* pKVM invalid pte encodings */
 #define KVM_INVALID_PTE_TYPE_MASK	GENMASK(63, 60)
 #define KVM_INVALID_PTE_ANNOT_MASK	~(KVM_PTE_VALID | \
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index b74dd5ce1efd..8c2320c6a7f1 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -730,11 +730,11 @@ static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot p
 
 	if (prot & KVM_PGTABLE_PROT_R)
 		attr |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R;
 
 	if (prot & KVM_PGTABLE_PROT_W)
-		attr |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
+		attr |= KVM_PTE_LEAF_ATTR_HI_S2_DBM | KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
 
 	if (!kvm_lpa2_is_enabled())
 		attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S2_SH, sh);
 
 	attr |= KVM_PTE_LEAF_ATTR_LO_S2_AF;
@@ -1286,10 +1286,11 @@ static int stage2_update_leaf_attrs(struct kvm_pgtable *pgt, u64 addr,
 }
 
 int kvm_pgtable_stage2_wrprotect(struct kvm_pgtable *pgt, u64 addr, u64 size)
 {
 	return stage2_update_leaf_attrs(pgt, addr, size, 0,
+					KVM_PTE_LEAF_ATTR_HI_S2_DBM |
 					KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W,
 					NULL, NULL,
 					KVM_PGTABLE_WALK_IGNORE_EAGAIN);
 }
 
@@ -1366,11 +1367,11 @@ int kvm_pgtable_stage2_relax_perms(struct kvm_pgtable *pgt, u64 addr,
 
 	if (prot & KVM_PGTABLE_PROT_R)
 		set |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R;
 
 	if (prot & KVM_PGTABLE_PROT_W)
-		set |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
+		set |= KVM_PTE_LEAF_ATTR_HI_S2_DBM | KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
 
 	if (prot & KVM_PGTABLE_PROT_X) {
 		ret = stage2_set_xn_attr(prot, &xn);
 		if (ret)
 			return ret;
-- 
2.56.0