Re: [PATCH v4 3/6] KVM: arm64: Add auto DBM support for hardware dirty tracking

[Leonardo Bras]

On Thu, Jul 09, 2026 at 06:40:23PM +0800, Tian Zheng wrote:
> The DBM (Dirty Bit Modifier) attribute, introduced in ARMv8.1, enables
> hardware to automatically promote write-clean pages to write-dirty. This
> prevents the guest from being trapped in EL2 due to missing write
> permissions.
> 
> In this design, DBM is controlled by the page-table level flag
> KVM_PGTABLE_S2_DBM rather than per-PTE software flags. DBM is
> automatically set for writable non-device pages when the page-table has
> KVM_PGTABLE_S2_DBM flag, which is determined at MMU init time based on
> hardware capability.
> 
> The DBM bit is set in stage2_set_prot_attr() for initial mappings and
> hugepage splitting, and directly manipulated in
> kvm_pgtable_stage2_relax_perms() when removing write-protection. On
> W->RO downgrade, DBM is cleared to prevent hardware from silently
> upgrading RO+DBM back to W+dirty, which would bypass KVM's write
> tracking.
> 
> kvm_pgtable_stage2_pte_prot() does not extract the DBM bit back into
> enum kvm_pgtable_prot because DBM is a page-table policy determined by
> pgt->flags, not a per-PTE property. Callers should check
> pgt->flags & KVM_PGTABLE_S2_DBM instead.
> 
> This ensures DBM is consistently applied across all PTEs, including
> during hugepage splitting where child PTEs inherit DBM from the parent
> block entry via the pgt->flags mechanism.
> 
> Safety: DBM bit is only interpreted by hardware when VTCR_EL2.HD=1.
> When HDBSS is not enabled (HD=0), ARM architecture guarantees hardware
> completely ignores DBM bit in PTEs.
> 
> Co-developed-by: Eillon <yezhenyu2@xxxxxxxxxx>
> Signed-off-by: Eillon <yezhenyu2@xxxxxxxxxx>
> Co-developed-by: Leonardo Bras <leo.bras@xxxxxxx>
> Signed-off-by: Leonardo Bras <leo.bras@xxxxxxx>

Hello Tian,

Have you added the above tags due to this patch being based on the below?
https://lore.kernel.org/all/20260629111820.1873540-2-leo.bras@xxxxxxx/

Thanks!
Leo


> Signed-off-by: Tian Zheng <zhengtian10@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h |  4 ++++
>  arch/arm64/kvm/hyp/pgtable.c         | 35 ++++++++++++++++++++++++++--
>  arch/arm64/kvm/mmu.c                 |  3 +++
>  3 files changed, 40 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 41a8687938eb..5e0fac4bfa53 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -93,6 +93,8 @@ 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 | \
> @@ -249,10 +251,12 @@ struct kvm_pgtable_mm_ops {
>   * enum kvm_pgtable_stage2_flags - Stage-2 page-table flags.
>   * @KVM_PGTABLE_S2_IDMAP:	Only use identity mappings.
>   * @KVM_PGTABLE_S2_AS_S1:	Final memory attributes are that of Stage-1.
> + * @KVM_PGTABLE_S2_DBM:	Hardware-managed DBM for writable pages.
>   */
>  enum kvm_pgtable_stage2_flags {
>  	KVM_PGTABLE_S2_IDMAP			= BIT(0),
>  	KVM_PGTABLE_S2_AS_S1			= BIT(1),
> +	KVM_PGTABLE_S2_DBM			= BIT(2),
>  };
> 
>  /**
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 91a7dfad6686..21ec456ecc41 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -731,9 +731,23 @@ 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)
> +	if (prot & KVM_PGTABLE_PROT_W) {
>  		attr |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
> 
> +		/*
> +		 * Set DBM bit for writable non-device pages if the page-table
> +		 * has KVM_PGTABLE_S2_DBM flag (system supports HDBSS).
> +		 *
> +		 * For stage 2 translations using Indirect permissions, if the
> +		 * Effective value of VTCR_EL2.HD is 0, then dirty state is
> +		 * managed by software. Hardware only updates the dirty state
> +		 * when VTCR_EL2.HD=1 (HDBSS enabled).
> +		 */
> +		if ((pgt->flags & KVM_PGTABLE_S2_DBM) &&
> +		    !(prot & KVM_PGTABLE_PROT_DEVICE))
> +			attr |= KVM_PTE_LEAF_ATTR_HI_S2_DBM;
> +	}
> +
>  	if (!kvm_lpa2_is_enabled())
>  		attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S2_SH, sh);
> 
> @@ -1367,9 +1381,26 @@ 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)
> +	if (prot & KVM_PGTABLE_PROT_W) {
>  		set |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
> 
> +		/*
> +		 * No DEVICE filter needed here: relax_perms is only called
> +		 * on FSC_PERM faults. Device pages always get full RW from
> +		 * initial mapping and are never write-protected during
> +		 * migration, so they never trigger a permission fault.
> +		 */
> +		if (pgt->flags & KVM_PGTABLE_S2_DBM)
> +			set |= KVM_PTE_LEAF_ATTR_HI_S2_DBM;
> +	} else {
> +		/*
> +		 * Clear DBM on W→RO downgrade to prevent hardware from
> +		 * silently upgrading RO+DBM back to W+dirty, which would
> +		 * bypass KVM's write tracking and cause data corruption.
> +		 */
> +		clr |= KVM_PTE_LEAF_ATTR_HI_S2_DBM;
> +	}
> +
>  	ret = stage2_set_xn_attr(prot, &xn);
>  	if (ret)
>  		return ret;
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index e086c01a9325..346efed6e605 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1014,6 +1014,9 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
>  	if (err)
>  		goto out_free_pgtable;
> 
> +	if (system_supports_hdbss())
> +		pgt->flags |= KVM_PGTABLE_S2_DBM;
> +
>  	mmu->pgt = pgt;
>  	if (is_protected_kvm_enabled())
>  		return 0;
> --
> 2.33.0
>