[PATCH v4 0/6] Support the FEAT_HDBSS introduced in Armv9.5
Tian Zheng
zhengtian10 at huawei.com
Tue Jul 14 06:29:27 PDT 2026
On 7/14/2026 7:20 PM, Leonardo Bras wrote:
> On Tue, Jul 14, 2026 at 06:39:48PM +0800, Tian Zheng wrote:
>> 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.
>>
>>
>
> Thanks for sharing :)
>
> Tip: for next version, you can automate that by adding
> '--base=<base-commit>' to your git-format-patch command, and it will
> automatically generate a base message at the end of your cover letter, such
> as:
>
> ```
> base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
> ```
>
> Thanks!
> Leo
Got it! thx!
>
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