Re: [PATCH v3 0/5] KVM: arm64: nv: Implement nested stage-2 reverse map

[Itaru Kitayama]

Hi Wei-Lin,

On Sun, May 10, 2026 at 03:53:33PM +0100, Wei-Lin Chang wrote:
> Hi,
> 
> This is v3 of optimizing the shadow s2 mmu unmapping during MMU
> notifiers.
> 
> Two new preparatory patches are added, one reduces a hole in kvm_s2_mmu
> and another refactors the code a bit during s2 faults. Other changes are
> listed below.
> 
> * Changes from v2 [1]:
> 
>   - Removed "polluted" teminology.
> 
>   - Use xa_{mk, to}_value() when storing and retriving values from maple
>     trees.
> 
>   - Avoid using the 63rd bit in maple tree values so that xa_{mk, to}_value()
>     does not lose us a bit.
> 
>   - Added reverse map removal during TLBI handling.
> 
>   - Other suggested refactorings.
> 
> Thanks!
> 
> [1]: https://lore.kernel.org/kvmarm/20260411125024.3735989-1-weilin.chang@xxxxxxx/
> 
> Wei-Lin Chang (5):
>   KVM: arm64: Use a variable for the canonical GPA in kvm_s2_fault_map()
>   KVM: arm64: Move shadow_pt_debugfs_dentry to reduce holes in
>     kvm_s2_mmu
>   KVM: arm64: nv: Avoid full shadow s2 unmap
>   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   |  17 +-
>  arch/arm64/include/asm/kvm_nested.h |   6 +
>  arch/arm64/kvm/mmu.c                |  43 +++--
>  arch/arm64/kvm/nested.c             | 238 +++++++++++++++++++++++++++-
>  arch/arm64/kvm/sys_regs.c           |   3 +
>  5 files changed, 290 insertions(+), 17 deletions(-)

While investigating the long VM teardown time on QEMU in the nested shadow 
stage 2 selftest, I instrumented a few functions with bpftrace.

The same kselftest workload produced the following counts:

Without this series (today's kvmarm/fixes):

kvm_nested_s2_unmap: 12
kvm_pgtable_stage2_unmap: 8,388,623
kvm_tlb_flush_vmid_range: 8,388,623
exit_vmid_context: 8,388,639

With this series:

kvm_nested_s2_unmap: 12
kvm_pgtable_stage2_unmap: 18
kvm_tlb_flush_vmid_range: 18
exit_vmid_context: 34

The bpftrace script used was:

#!/usr/bin/env bpftrace

fentry:kvm_nested_s2_unmap
{
    @nested = count();
}

fentry:kvm_pgtable_stage2_unmap
{
    @pgunmap = count();
}

fentry:kvm_tlb_flush_vmid_range
{
    @flush = count();
}

fentry:exit_vmid_context
{
    @exit = count();
}

END
{
    print(@nested);
    print(@pgunmap);
    print(@flush);
    print(@exit);
}

Is this consistent with what you evaluated during the development?
I wonder if this is good for nested guest boot paths too.

Thanks,
Itaru.

> 
> -- 
> 2.43.0
>