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

[Tian Zheng <zhengtian10@huawei.com>]

On 7/14/2026 12:27 AM, Leonardo Bras wrote:
> On Mon, Jul 13, 2026 at 11:31:46AM +0100, 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.
>>
>>> 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
>>>
> Also, please remember to include the base commit, so it can be applied for
> testing/reviewing.


Thank you for the reminder.

This patch series is based on:
- Commit: 0e35b9b6ec0f
- Description: Merge tag 'mm-hotfixes-stable-2026-07-06-17-49' of 
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Welcome to test and review — please let me know if you encounter any 
issues or have suggestions for improvement.