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

[Leonardo Bras <leo.bras@arm.com>]

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