Re: [PATCH v5 01/15] KVM: arm64: pgtables: Change write bit from S2AP_W to DBM

[Oliver Upton]

Hi Tian,

On Tue, Sep 29, 2026 at 06:36:41PM +0800, Tian Zheng wrote:
> From: Leonardo Bras <leo.bras@xxxxxxx>
> 
> 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/20260901171558.2674031-2-leo.bras@xxxxxxx/
> Signed-off-by: Leonardo Bras <leo.bras@xxxxxxx>
> [zhengtian: keep the nested walker reading writability from S2AP[1]
> alone, document why, and reword the commit message]
> Signed-off-by: Tian Zheng <zhengtian10@xxxxxxxxxx>
> ---
>  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.

> 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
>