Re: [RFC PATCH 1/5] KVM: arm64: pgtables: Change write bit from S2AP_W to DBM

[Marc Zyngier <maz@kernel.org>]

On Tue, 01 Sep 2026 18:15:52 +0100,
Leonardo Bras <leo.bras@arm.com> wrote:
> 
> 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).
> 
> For this step, we convert usages of RW(Dirty) -> WD(DBM|Dirty).
> 
> Signed-off-by: Leonardo Bras <leo.bras@arm.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h | 3 +++
>  arch/arm64/kvm/hyp/pgtable.c         | 8 +++++---
>  arch/arm64/kvm/nested.c              | 4 +++-
>  arch/arm64/kvm/ptdump.c              | 4 ++--
>  4 files changed, 13 insertions(+), 6 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
> @@ -86,24 +86,27 @@ typedef u64 kvm_pte_t;
>  #define KVM_PTE_LEAF_ATTR_HI		GENMASK(63, 50)
>  
>  #define KVM_PTE_LEAF_ATTR_HI_SW		GENMASK(58, 55)
>  
>  #define KVM_PTE_LEAF_ATTR_HI_S1_XN	BIT(54)
>  #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 | \
>  					  KVM_INVALID_PTE_TYPE_MASK)
>  
>  enum kvm_invalid_pte_type {
>  	/*
>  	 * Used to indicate a pte for which a 'break-before-make'
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index b74dd5ce1efd..ca49f1bd7c34 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -725,42 +725,43 @@ static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot p
>  	}
>  
>  	r = stage2_set_xn_attr(prot, &attr);
>  	if (r)
>  		return r;
>  
>  	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;

What makes it acceptable to always set DBM? This is an optional
feature, and I'm not exactly comfortable setting bits that are
supposed to be RES0. Yes, the HW should ignore it. But we have also
seen quite a few broken designs in this area...

> +
>

Spurious newline.

>  	if (!kvm_lpa2_is_enabled())
>  		attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S2_SH, sh);
>  
>  	attr |= KVM_PTE_LEAF_ATTR_LO_S2_AF;
>  	attr |= prot & KVM_PTE_LEAF_ATTR_HI_SW;
>  	*ptep = attr;
>  
>  	return 0;
>  }

I don't know how you have configured git on your end, but there is *a
lot* of context...

>  
>  enum kvm_pgtable_prot kvm_pgtable_stage2_pte_prot(kvm_pte_t pte)
>  {
>  	enum kvm_pgtable_prot prot = pte & KVM_PTE_LEAF_ATTR_HI_SW;
>  
>  	if (!kvm_pte_valid(pte))
>  		return prot;
>  
>  	if (pte & KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R)
>  		prot |= KVM_PGTABLE_PROT_R;
> -	if (pte & KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W)
> +	if (pte & KVM_PTE_LEAF_ATTR_HI_S2_DBM)
>  		prot |= KVM_PGTABLE_PROT_W;
>  
>  	switch (FIELD_GET(KVM_PTE_LEAF_ATTR_HI_S2_XN, pte)) {
>  	case 0b00:
>  		prot |= KVM_PGTABLE_PROT_PX | KVM_PGTABLE_PROT_UX;
>  		break;
>  	case 0b01:
>  		prot |= KVM_PGTABLE_PROT_UX;
>  		break;
>  	case 0b11:
> @@ -1281,20 +1282,21 @@ static int stage2_update_leaf_attrs(struct kvm_pgtable *pgt, u64 addr,
>  		*orig_pte = data.pte;
>  
>  	if (level)
>  		*level = data.level;
>  	return 0;
>  }
>  
>  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);
>  }
>  
>  void kvm_pgtable_stage2_mkyoung(struct kvm_pgtable *pgt, u64 addr,
>  				enum kvm_pgtable_walk_flags flags)
>  {
>  	int ret;
>  
> @@ -1361,21 +1363,21 @@ int kvm_pgtable_stage2_relax_perms(struct kvm_pgtable *pgt, u64 addr,
>  	s8 level;
>  	int ret;
>  
>  	if (prot & KVM_PTE_LEAF_ATTR_HI_SW)
>  		return -EINVAL;
>  
>  	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;
>  
>  		set |= xn & KVM_PTE_LEAF_ATTR_HI_S2_XN;
>  		clr |= ~xn & KVM_PTE_LEAF_ATTR_HI_S2_XN;
>  	}
>  
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index 17123f0b6dab..eb8dfffc32c7 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -379,21 +379,23 @@ static int walk_nested_s2_pgd(struct kvm_vcpu *vcpu, phys_addr_t ipa,
>  	}
>  
>  	addr_bottom += contiguous_bit_shift(desc, wi, level);
>  
>  	/* Calculate and return the result */
>  	paddr = (desc & GENMASK_ULL(47, addr_bottom)) |
>  		(ipa & GENMASK_ULL(addr_bottom - 1, 0));
>  	out->output = paddr;
>  	out->block_size = 1UL << ((3 - level) * stride + wi->pgshift);
>  	out->readable = desc & KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R;
> -	out->writable = desc & KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
> +	/* Takes care of both RO/RW and RO/WC/WD encodings */
> +	out->writable = desc & (KVM_PTE_LEAF_ATTR_HI_S2_DBM |
> +				KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W);

Absolutely NOT. For a start, NV doesn't support FEAT_HAFDBS. But even
if it did, you are now actively corrupting memory by turning a RO
mapping with a spurious DBM bit set into a writable mapping.
VTCR_EL2.HD exists for a reason.

Do you see why your blanket approach of equating DBM with writable is
plain wrong?

	M.

-- 
Without deviation from the norm, progress is not possible.