On 9/30/2026 8:25 AM, Oliver Upton wrote:
Hi Tian,
On Tue, Sep 29, 2026 at 06:36:41PM +0800, Tian Zheng wrote:
From: Leonardo Bras <[email protected]>
As a first step of changing the encoding for the Stage2 PTE descriptor,
introduce the DBM bit, and adapt every usage of writable to use the DBM
bit (51) instead of S2AP[1]/Dirty bit (7).
With DBM as the write permission bit and S2AP[1] as the dirty state,
the encoding follows the FEAT_S2PIE principle of managing permissions
and dirty state independently.
For this step, we convert usages of RW(Dirty) -> WD(DBM|Dirty): every
writable mapping sets both bits, read-only mappings clear both, and no
behaviour changes.
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Leonardo Bras <[email protected]>
[zhengtian: keep the nested walker reading writability from S2AP[1]
alone, document why, and reword the commit message]
Signed-off-by: Tian Zheng <[email protected]>
---
arch/arm64/include/asm/kvm_pgtable.h | 3 +++
arch/arm64/kvm/hyp/pgtable.c | 7 ++++---
arch/arm64/kvm/nested.c | 5 +++++
arch/arm64/kvm/ptdump.c | 4 ++--
4 files changed, 14 insertions(+), 5 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
@@ -93,10 +93,13 @@ typedef u64 kvm_pte_t;
#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 */
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index b74dd5ce1efd..50f4d3a74f77 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -732,7 +732,7 @@ static int stage2_set_prot_attr(struct kvm_pgtable *pgt,
enum kvm_pgtable_prot p
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;
Hmm... I'd actually like to see this structured where the "write" bit
could either be S2AP_W or DBM depending on if the system supports
FEAT_HAFDBS. Look at how HVHE is handled for selecting the right AP
encoding for the hyp stage-1 page tables.
Hi Oliver,
Thanks for the suggestion. I missed the case where FEAT_HAFDBS is not
supported. Systems without the feature should keep the original
encoding, with S2AP[1] as the write permission bit. I'll follow the HVHE
pattern to select between the two encodings in the next version.
Thanks!
Tian
diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c
index 69899797dbad..b0cb8d84a9e9 100644
--- a/arch/arm64/kvm/ptdump.c
+++ b/arch/arm64/kvm/ptdump.c
@@ -40,8 +40,8 @@ static const struct ptdump_prot_bits stage2_pte_bits[] = {
.clear = " ",
},
{
- .mask = KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W,
- .val = KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W,
+ .mask = KVM_PTE_LEAF_ATTR_HI_S2_DBM,
+ .val = KVM_PTE_LEAF_ATTR_HI_S2_DBM,
.set = "W",
.clear = " ",
},
--
2.43.0
