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

[Tian Zheng <zhengtian10@huawei.com>]

On 7/14/2026 3:23 PM, Marc Zyngier wrote:
> On Tue, 14 Jul 2026 02:14:45 +0100,
> Tian Zheng <zhengtian10@huawei.com> wrote:
>>
>> On 7/13/2026 7:17 PM, Leonardo Bras wrote:
>>> 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@huawei.com>
>>>> Signed-off-by: Eillon <yezhenyu2@huawei.com>
>>>> Co-developed-by: Leonardo Bras <leo.bras@arm.com>
>>>> Signed-off-by: Leonardo Bras <leo.bras@arm.com>
>>> 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@arm.com/
>>>
>>> Thanks!
>>> Leo
>> Hi Leo,
>>
>>
>> Yes, I added your Signed-off-by because the DBM-related code in this patch
> You really can't do that. Only Leo can give his SoB, you can't forge
> it yourself.
>
>> is based on your implementation in:
>>
>> https://lore.kernel.org/all/20260629111820.1873540-2-leo.bras@arm.com/
> Then take the patch as is, and add to it as a separate patch. Or work
> out in private with Leo whether he's happy with a Co-dev. But never do
> that unilaterally.
>
> Thanks,
>
> 	M.
Got it, thanks for the clarification, Marc.

I'm sorry about that — I'll remove them in the next version.
Instead, I'll mention the reference in the commit message with a link to 
Leo's original patch:

Based on Leonardo Bras's patch:
https://lore.kernel.org/all/20260629111820.1873540-2-leo.bras@arm.com/