Re: [PATCH v4 0/6] Support the FEAT_HDBSS introduced in Armv9.5

[Tian Zheng <zhengtian10@huawei.com>]

On 7/13/2026 6:31 PM, Leonardo Bras wrote:
> On Thu, Jul 09, 2026 at 06:40:20PM +0800, Tian Zheng wrote:
>> This series of patches add support to the Hardware Dirty state tracking
>> Structure (HDBSS) feature, which is introduced by the ARM architecture
>> in the DDI0601 (ID121123) version.
>>
>> The HDBSS feature is an extension to the architecture that enhances
>> tracking translation table descriptors' dirty state, identified as
>> FEAT_HDBSS. This feature utilizes hardware assistance to achieve dirty
>> page tracking, aiming to significantly reduce the overhead of scanning
>> for dirty pages.
>>
>> The purpose of this feature is to make the execution overhead of live
>> migration lower to both the guest and the host, compared to existing
>> approaches (write-protect or search stage-2 tables).
>>
>> The required sysreg definitions for FEAT_HDBSS have been merged into
>> arm64 /sysregs:
>>    [1/5] arm64/sysreg: Add HDBSS related register information
>>          https://git.kernel.org/arm64/c/72f7be0c2e30
>>
>>
>> After these patches, the kernel automatically enables HDBSS when dirty
>> logging is enabled on any memslot, and disables HDBSS when dirty logging
>> is disabled on all memslots. This series does not support dirty ring
>> mode.
>>
>> Depends-on: "KVM: arm64: Enable eager hugepage splitting if HDBSS is available"
>> 	https://lore.kernel.org/linux-arm-kernel/20260629111820.1873540-3-leo.bras@arm.com/
> On this, FYI, there have been some discussion on this:
> https://lore.kernel.org/all/alETGFD2Ogx6N0HB@LeoBrasDK/
>
> Oliver's suggestion is that we don't automatically enable eager splitting,
> but instead we have different behaviours if the user enables it.
>
> This is still under discussion there, but I think it can be useful reading.


Thanks for the pointer — I've read through the discussion between you 
and Oliver, and it's very helpful.


If I understand correctly, the proposed approach is to support both 
modes, with DBM behavior depending

on the user's eager split setting:

- If users enable eager splitting (chunk_size != 0): use the v4 approach 
— DBM is set globally.

- If users do NOT enable eager splitting (chunk_size == 0, i.e., lazy 
split mode): use the v3 approach — DBM

is set lazily in user_mem_abort on the first write fault.


This makes sense to me. It gives users the flexibility to choose.


As I mentioned in my patch 6/6 reply 
(https://lore.kernel.org/all/6e8b23bc-d420-4f5a-a921-5a5d64d84200@huawei.com/),

I do think the lazy DBM approach is safer overall if the first-fault 
overhead is acceptable — it avoids accidentally marking

special mappings and naturally handles lazy split.


I'll keep an eye on the discussion and follow up once a conclusion is 
reached.


Thanks!

Tian

>> This dependency is mandatory. Unlike v3, where DBM was added lazily on
>> the first dirty access via a page fault, v4 injects DBM into pgt->flags
>> at stage-2 MMU initialization time. Combined with HDBSS being auto-enabled
>> during migration setup, the first dirty access no longer triggers a page
>> fault. Consequently, if lazy hugepage splitting were relied upon (which
>> requires a page fault to trigger splitting), hugepages would never be
>> split, leading to guest hang after migration. Leonardo's patch above
>> ensures eager hugepage splitting is enabled (chunk_size != 0) when HDBSS
>> is available, guaranteeing that hugepages are properly split before
>> migration starts.
>>
>> v3: https://lore.kernel.org/linux-arm-kernel/20260225040421.2683931-1-zhengtian10@huawei.com/
>>
>> v3->v4 changes:
>> - Merge sysreg definitions into the FEAT_HDBSS detection patch (was a
>>    separate patch in v3).
>> - Add auto DBM (Dirty Bit Modifier) support as a new patch, suggested
>>    by Leonardo Bras. DBM is now controlled as a page-table level flag
>>    (KVM_PGTABLE_S2_DBM) rather than per-PTE. Note that DBM is injected
>>    at stage-2 MMU creation time, not lazily on first dirty access. This
>>    means the first write to a dirty-logged page does not generate a
>>    page fault, which is a key reason for the mandatory dependency on
>>    Leonardo's eager hugepage splitting patch (see Depends-on above).
>> - Split the v3 "Enable HDBSS support and handle HDBSSF events" patch
>>    into three patches: per-vCPU buffer management, fault handling and
>>    buffer flush, and auto enable/disable on dirty logging change. This
>>    implements kernel-managed automatic HDBSS enable/disable.
>> - Remove the KVM_CAP_ARM_HW_DIRTY_STATE_TRACK ioctl for manual HDBSS
>>    on/off. HDBSS is now automatically enabled/disabled based on dirty
>>    logging state via kvm_arch_commit_memory_region().
>> - Change HDBSS buffer flush triggers to vcpu_put, check_vcpu_requests,
>>    and kvm_handle_guest_abort.
>> - Store hdbss_order at VM level (kvm->arch.hdbss_order) instead of
>>    per-vCPU, since all vCPUs share the same order.
>> - Document patch is not included in this version; will be sent in a
>>    follow-up series.
>>
>> Leonardo Bras (1):
>>    KVM: arm64: Enable eager hugepage splitting if HDBSS is available
>>
>> Tian Zheng (2):
>>    KVM: arm64: Add support for FEAT_HDBSS
>>    KVM: arm64: Add auto DBM support for hardware dirty tracking
>>
>> eillon (3):
>>    KVM: arm64: Add HDBSS per-vCPU buffer management
>>    KVM: arm64: Add HDBSS fault handling and buffer flush
>>    KVM: arm64: Add auto HDBSS enable/disable on dirty logging change
>>
>>   arch/arm64/include/asm/cpufeature.h    |   5 +
>>   arch/arm64/include/asm/esr.h           |   5 +
>>   arch/arm64/include/asm/kvm_dirty_bit.h |  29 ++++
>>   arch/arm64/include/asm/kvm_host.h      |  14 ++
>>   arch/arm64/include/asm/kvm_pgtable.h   |   4 +
>>   arch/arm64/include/asm/sysreg.h        |  11 ++
>>   arch/arm64/kernel/cpufeature.c         |  12 ++
>>   arch/arm64/kvm/Makefile                |   1 +
>>   arch/arm64/kvm/arm.c                   |  29 ++++
>>   arch/arm64/kvm/dirty_bit.c             | 219 +++++++++++++++++++++++++
>>   arch/arm64/kvm/hyp/pgtable.c           |  35 +++-
>>   arch/arm64/kvm/hyp/vhe/switch.c        |  15 ++
>>   arch/arm64/kvm/mmu.c                   |  19 ++-
>>   arch/arm64/kvm/reset.c                 |   4 +
>>   arch/arm64/tools/cpucaps               |   1 +
>>   15 files changed, 399 insertions(+), 4 deletions(-)
>>   create mode 100644 arch/arm64/include/asm/kvm_dirty_bit.h
>>   create mode 100644 arch/arm64/kvm/dirty_bit.c
>>
>> --
>> 2.33.0
>>