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

[Tian Zheng <zhengtian10@huawei.com>]

On 7/17/2026 11:21 PM, Leonardo Bras wrote:
> On Fri, Jul 17, 2026 at 11:58:06AM +0800, Tian Zheng wrote:
>>
>> On 7/16/2026 3:39 PM, Oliver Upton wrote:
>>> Hi Tian,
>>>
>>> On Thu, Jul 09, 2026 at 06:40:23PM +0800, Tian Zheng wrote:
>>>> -	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;
>>>> +	}
>>>> +
>>> This block makes it pretty evident that the DBM bit really *is* the
>>> write permission bit. I'd much rather we introduce the concept of dirty
>>> state to the page table library and migrate the abstract write
>>> permission to the DBM field, even if we don't have FEAT_HAFDBS.
>>>
> 
> Ohh, that's an amazing idea!
> 
>   
>>> That way everything 'just works' from outside the page-table library:
>>> write-protecting hugepages would have the effect of clearing DBM and we
>>> can separately reap dirty state from page descriptors.
>>>
>>> If/when the architecture forces FEAT_S2PIE upon us we will need to make
>>> this change anyway since dirty state management is unconditional and
>>> handled separately from the actual permissions.
>>>
>>> Thanks,
>>> Oliver
>>
>> Hi Oliver,
>>
>> Thanks again for your insightful review. Following your suggestion, I've
>>
>> reworked the design around a unified three-state model that works regardless
>>
>> of whether FEAT_HAFDBS is implemented:
>>
>> **State table**
>> State               | DBM    | S2AP[1] | Without HTTU            | With  HTTU  (HAFDBS)
>> Non-writable   (N)  |  0     |    0    | write -> fault, inject  | write -> fault, inject
>> Writable-clean (C)  |  1     |    0    | write -> fault, sw C->D | write -> hw C->D, no fault, HDBSS logs
>> Writable-dirty (D)  |  1     |    1    | writable, no fault      | writable, no fault
>>
> 
> Yeah, that's how the table works with HAFDBS/HDBSS/HACDBS.
> 
>> **Proposed changes**
>> 1. Remove KVM_PGTABLE_S2_DBM from enum kvm_pgtable_stage2_flags
>>
>> — VTCR_EL2.{HD,HDBSS,HA} enablement in kvm_arm_enable_hdbss_global()
>>
>> already keys off kvm->arch.enable_hdbss / system_supports_hdbss().
>>
> 
> We may need a system_support_hdbss() for the actual hdbss routines, though.

Yes, and sorry for the unclear phrasing. What I meant is:

Setting DBM unconditionally is safe because hardware only interprets it 
when VTCR_EL2.HD is set. That only happens in 
kvm_arm_enable_hdbss_global() after checking both 
system_supports_hdbss() and migration state.

> 
> 
>> 2. stage2_set_prot_attr() — set DBM unconditionally on writable pages:
>> ```
>> if (prot & KVM_PGTABLE_PROT_W) {
>>      attr |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
>>      /* Writable-dirty: DBM=1 conveys write intent, S2AP[1]=1 marks dirty */
>>      attr |= KVM_PTE_LEAF_ATTR_HI_S2_DBM;
>> }
>> ```
>>
>> 3. kvm_pgtable_stage2_relax_perms() — drop the else branch entirely:
>> ```
>> if (prot & KVM_PGTABLE_PROT_W) {
>>      set |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
>>      /* Non-writable -> Writable-dirty: restore both write intent and dirty state */
>>      set |= KVM_PTE_LEAF_ATTR_HI_S2_DBM;
> 
> In the future, depending on the setup of HDBSS/splitting, we may want to
> change this behavior. But for software only, it looks nice.
> 
>> }
>> /* no else: callers passing !W (e.g. exec faults) must not touch DBM */
>> ```
>>
>> 4. kvm_pgtable_stage2_wrprotect() — unchanged: it only clears S2AP1 (D->C).
>>
>> DBM is preserved so HDBSS re-arms next round.
>>   ```
>> int kvm_pgtable_stage2_wrprotect(struct kvm_pgtable *pgt, u64 addr, u64
>> size)
>> {
>>     /* Writable-dirty -> Writable-clean: clear dirty state (S2AP_W),
>>     * preserve write intent (DBM) so HDBSS re-arms for next write.
>>      */
>>      return stage2_update_leaf_attrs(pgt, addr, size, 0,
>>                           KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W,
>>                           NULL, NULL,
>>                           KVM_PGTABLE_WALK_IGNORE_EAGAIN);
>> }
>> ```
>>
>> **One clarification**
>> In the three-state model above, wrprotect() clears S2AP[1] but preserves DBM
>> (D->C).
>>
>> This allows HDBSS to re-arm on the next write. If we instead cleared DBM as
>> well (->N),
>>
>> HDBSS would be permanently disabled on that page and we'd lose the benefit
>> of hardware
>>
>> dirty tracking.
>>
>>
>> So my understanding is:
>>
>> wrprotect() (dirty tracking): D->C — clears S2AP[1], preserves DBM
>>
>> mkreadonly() (true RO, future): ->N — clears both S2AP[1] and DBM
>>
>> Does this match what you had in mind?
>>
>> Looking forward to your thoughts.
>>
>> Thanks,
>> Tian
>>
>>
>   
> Thanks!
> Leo
>