Subject: Re: [PATCH v3 0/5] Support the FEAT_HDBSS introduced in Armv9.5

From: Leonardo Bras <[email-masked]>
Date: Tue, 31 Mar 2026 15:13:50 +0100

On Wed, Feb 25, 2026 at 12:04:16PM +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).
> 
> After these patches, users(such as qemu) can use the
> KVM_CAP_ARM_HW_DIRTY_STATE_TRACK ioctl to enable or disable the HDBSS
> feature before and after the live migration.
> 
> v2:
> https://lore.kernel.org/linux-arm-kernel/[email-masked]/
> 
> v2->v3 changes:
> - Remove the ARM64_HDBSS configuration option and ensure this feature
> is only enabled in VHE mode.
> - Move HDBSS-related variables to the arch-independent portion of the
> kvm structure.
> - Remove error messages during HDBSS enable/disable operations
> - Change HDBSS buffer flushing from handle_exit to vcpu_put,
> check_vcpu_requests, and kvm_handle_guest_abort.
> - Add fault handling for HDBSS including buffer full, external abort,
> and general protection fault (GPF).
> - Add support for a 4KB HDBSS buffer size, mapped to the value 0b0000.
> - Add a second argument to the ioctl to turn HDBSS on or off.
> 
> Tian Zheng (1):
>   KVM: arm64: Document HDBSS ioctl
> 
> eillon (4):
>   arm64/sysreg: Add HDBSS related register information
>   KVM: arm64: Add support to set the DBM attr during memory abort
>   KVM: arm64: Add support for FEAT_HDBSS
>   KVM: arm64: Enable HDBSS support and handle HDBSSF events
> 
>  Documentation/virt/kvm/api.rst       |  16 +++++
>  arch/arm64/include/asm/cpufeature.h  |   5 ++
>  arch/arm64/include/asm/esr.h         |   7 ++
>  arch/arm64/include/asm/kvm_host.h    |  17 +++++
>  arch/arm64/include/asm/kvm_mmu.h     |   1 +
>  arch/arm64/include/asm/kvm_pgtable.h |   4 ++
>  arch/arm64/include/asm/sysreg.h      |  11 +++
>  arch/arm64/kernel/cpufeature.c       |  12 ++++
>  arch/arm64/kvm/arm.c                 | 102 +++++++++++++++++++++++++++
>  arch/arm64/kvm/hyp/pgtable.c         |   6 ++
>  arch/arm64/kvm/hyp/vhe/switch.c      |  19 +++++
>  arch/arm64/kvm/mmu.c                 |  70 ++++++++++++++++++
>  arch/arm64/kvm/reset.c               |   3 +
>  arch/arm64/tools/cpucaps             |   1 +
>  arch/arm64/tools/sysreg              |  29 ++++++++
>  include/uapi/linux/kvm.h             |   1 +
>  tools/include/uapi/linux/kvm.h       |   1 +
>  17 files changed, 305 insertions(+)
> 
> --
> 2.33.0
> 

Hi Tian,

I was thinking: maybe instead of putting the HDBSS (and HACDBS) stuff 
across a bunch of KVM files, we should try to focus them all on a single 
arch/arm64/kvm/dirty_bit.c file (plus a header such as 
arch/arm64/include/asm/kvm_dirty_bit.h).

What is your opinion on that?

Thanks!
Leo