Re: [PATCH v4 0/6] KVM: arm64: nv: Implement nested stage-2 reverse map

[Itaru Kitayama <itaru.kitayama@fujitsu.com>]

On Tue, Jul 14, 2026 at 12:59:19PM +0100, Wei-Lin Chang wrote:
> Hi,
> 
> This is v4 of optimizing the shadow s2 mmu unmapping during MMU
> notifiers. The general theme stays the same, but various
> improvements/fixes are added. Thanks for all the feedback. Please see
> below for the changes.
> 
> * Changes from v3 [1]:
> 
>   - Rebased on to the latest kvmarm/fixes.
> 
>   - Removed commit "KVM: arm64: Move shadow_pt_debugfs_dentry to reduce
>     holes in kvm_s2_mmu" because the current shadow ptdump code is found
>     to be buggy and the proposed fix removes shadow_pt_debugfs_dentry [2].
> 
>   - Rename some variables to improve readability e.g. ipa ->
>     canonical_ipa, fault_ipa -> nested_ipa.
> 
>   - Skip reverse map optimization when !mmu->nested_stage2_enabled,
>     and just unmap the canonical IPA range during MMU notifiers.
> 
>   - Change comment above the code that checks whether an identical
>     reverse map already exists in kvm_record_nested_revmap() from saying
>     it could be a permission update to saying another parallel stage-2
>     fault could have inserted an identical reverse map. This is because
>     permission update won't trigger kvm_record_nested_revmap().
> 
>   - Add helpers for checking maple tree entry flags VALID_ENTRY and
>     UNKNOWN_IPA, also warn if both are set at the same time.
> 
>   - Test canonical IPA alignment with the mapping size during reverse
>     map insertion.
> 
>   - Move clean up of kvm_unmap_gfn_range() to its own patch.
> 
>   - Avoid naming variables with "*dir*", as it can be misunderstood to
>     be related to directories.
> 
>   - Use mas_find() instead of mas_find_range() as mas_find_range()
>     returns every range including empty/NULL ranges, for our use case
>     we only want to iterate over non-NULL ranges.
> 
>   - Added a trace point to track when the reverse map maple tree becomes
>     "broken" i.e. maple tree insertion failure.
> 
> Thanks!
> 
> [1]: https://lore.kernel.org/kvmarm/20260510145338.322962-1-weilin.chang@arm.com/
> [2]: https://lore.kernel.org/kvmarm/20260630121005.1130996-1-weilin.chang@arm.com/

Thanks for the update. I know this series is sitll being discussed, but I
went ahead testing the kernel with your proposed hello_nested seltest on a Grace
CPU. 

Before this series applied:
real    0m2.869s
user    0m0.003s
sys     0m2.862s

After this series applied:
real    0m0.027s
user    0m0.001s
sys     0m0.005s

It reduces the time needed for the VM tear-down paths. I will test this series again 
when v5 comes out, since I've git this a try;

Tested-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>

Thanks,
Itaru.

> 
> Wei-Lin Chang (6):
>   KVM: arm64: Use a variable for the canonical GPA in kvm_s2_fault_map()
>   KVM: arm64: nv: Avoid full shadow s2 unmap
>   KVM: arm64: nv: Add nested revmap broken tracepoint
>   KVM: arm64: Refactor kvm_unmap_gfn_range() with common variables
>   KVM: arm64: nv: Remove reverse map entries during TLBI handling
>   KVM: arm64: nv: Create nested IPA direct map to speed up reverse map
>     removal
> 
>  arch/arm64/include/asm/kvm_host.h   |   9 +
>  arch/arm64/include/asm/kvm_nested.h |   6 +
>  arch/arm64/kvm/mmu.c                |  42 +++--
>  arch/arm64/kvm/nested.c             | 280 +++++++++++++++++++++++++++-
>  arch/arm64/kvm/sys_regs.c           |   3 +
>  arch/arm64/kvm/trace_arm.h          |  27 +++
>  6 files changed, 349 insertions(+), 18 deletions(-)
> 
> -- 
> 2.43.0
>