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

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@arm.com/ (local)

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