Re: [PATCH 00/22] KVM: arm64: nv: Implement FEAT_HAFDBS, FEAT_HAFT

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

On Tue, Jun 23, 2026 at 11:41:39AM -0700, Oliver Upton wrote:
> KVM's support for hardware descriptor updates has been lacking, owing in
> large part to the fact that the most widely available NV2 hardware
> actually lacks HAFDBS :)
> 
> This series brings support for hardware dirty state and table Access
> flags to the nested MMU. While KVM uses neither of these features, the
> kernel definitely does in the stage-1 page tables. Since there is only
> one hypervisor to rule them all, implementing this stuff at stage-2 is
> mostly motivated by the desire to enable guest stage-1 :)
> 
> Implementing dirty state at stage-2 is a bit more challenging than
> anticipated, I'm in the middle of seeking a relaxation from Arm that
> would make this all architectural. See the last patch for the details.

Hi Oliver,

Haven't read your patchset yet, but in regards to stage-2 dirty-state
tracking, there have already been some efforts in the area, also
implementing HDBSS and HACDBS in a way of improving performance / reducing 
impact of live migration on a system.

HDBSS support is being done by Tian, and according to his last message, we
should have a v4 at any moment:
v3 link:
https://lore.kernel.org/all/20260225040421.2683931-1-zhengtian10@huawei.com/

I am working on the HACDBS implementation myself, and plan to send a v2 
next monday. (Just waiting the merge window to close)
v1 link:
https://lore.kernel.org/all/20260430111424.3479613-2-leo.bras@arm.com/

Thanks!
Leo


> 
> Applies to kvmarm/fixes since this depends on some of the earlier VNCR
> rework. Lightly tested on an implementation with HAFT.
> 
> Oliver Upton (22):
>   KVM: arm64: nv: Introduce struct for stage-2 walk step
>   KVM: arm64: nv: Consolidate computation of stage-2 permissions
>   KVM: arm64: nv: Get rid of kvm_s2_trans*() accessors
>   KVM: arm64: nv: Only shadow writable-dirty guest descs as writable
>   KVM: arm64: nv: Pass an access descriptor for stage-2 walks
>   KVM: arm64: nv: Use a helper for stage-2 descriptor updates
>   KVM: arm64: nv: Set dirty state at stage-2
>   KVM: arm64: nv: Treat DBM as writable at stage-2
>   KVM: arm64: Compute S1 permissions as part of s1_walk()
>   KVM: arm64: Plumb through access descriptor for stage-1
>   KVM: arm64: Use a struct for stage-1 walk context
>   KVM: arm64: Create helper for stage-1 descriptor updates
>   KVM: arm64: Set dirty state at stage-1
>   KVM: arm64: Grant write permission when DBM is set at S1
>   KVM: arm64: Don't update descriptors for "non-arch" access
>   KVM: arm64: nv: Expose FEAT_HAFDBS
>   KVM: arm64: Set Access flag on table descriptors at stage-1
>   KVM: arm64: nv: Set access flag on table descriptors at stage-2
>   KVM: arm64: nv: Expose FEAT_HAFT
>   KVM: arm64: selftests: Only test AF behavior for emulated AT insns
>   KVM: arm64: selftests: Test that AT emulation for FEAT_HAFT
>   HACK: KVM: arm64: nv: Set the dirty state for CMOs that fetch for
>     write
> 
>  arch/arm64/include/asm/kvm_arm.h       |   1 +
>  arch/arm64/include/asm/kvm_nested.h    |  94 +++------
>  arch/arm64/include/asm/kvm_pgtable.h   |  17 +-
>  arch/arm64/kvm/at.c                    | 267 +++++++++++++++++--------
>  arch/arm64/kvm/hyp/pgtable.c           |  11 -
>  arch/arm64/kvm/mmu.c                   |  36 +++-
>  arch/arm64/kvm/nested.c                | 251 ++++++++++++++++-------
>  tools/testing/selftests/kvm/arm64/at.c | 102 ++++++++--
>  8 files changed, 515 insertions(+), 264 deletions(-)
> 
> 
> base-commit: d098bb75d14fde2f12155f1a95ec0168160867ce
> -- 
> 2.47.3
>