Re: [PATCH v5 0/5] KVM: arm64: Add KVM_PRE_FAULT_MEMORY support

Re: [PATCH v5 0/5] KVM: arm64: Add KVM_PRE_FAULT_MEMORY support

[Alexandru Elisei]

Hi,

On Fri, Jun 12, 2026 at 05:23:48PM +0100, Jack Thomson wrote:
> From: Jack Thomson <jackabt@amazon.com>
> 
> Hi,
> 
> This series adds arm64 support for KVM_PRE_FAULT_MEMORY, which was added
> for x86 in [1]. The ioctl allows userspace to populate stage-2 mappings
> before running a vCPU, reducing the number of stage-2 faults taken in
> the run path. This is useful for post-copy migration, where stage-2
> fault latency shows up directly in memory-intensive workloads.
>
> ..
> 
> base-commit: 98f826f3c500fda08d51fca434b7aefa6a2f7076

I want to review and test the series, but I cannot seem to find the base
commit in the master branch:

$ git show --oneline
87320be9f0d2 (HEAD -> master, origin/master, origin/HEAD) Merge tag 'net-7.2-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net

$ git show 98f826f3c500fda08d51fca434b7aefa6a2
fatal: ambiguous argument '98f826f3c500fda08d51fca434b7aefa6a2': unknown revision or path not in the working tree.

Thanks,
Alex

[PATCH v5 0/5] KVM: arm64: Add KVM_PRE_FAULT_MEMORY support

[Jack Thomson]

From: Jack Thomson <jackabt@amazon.com>

Hi,

This series adds arm64 support for KVM_PRE_FAULT_MEMORY, which was added
for x86 in [1]. The ioctl allows userspace to populate stage-2 mappings
before running a vCPU, reducing the number of stage-2 faults taken in
the run path. This is useful for post-copy migration, where stage-2
fault latency shows up directly in memory-intensive workloads.

On arm64, the GPA supplied to the ioctl is treated as an IPA in the
userspace-owned VM's memslot address space. If the vCPU most recently
ran a nested guest, KVM still targets the VM's canonical stage-2. It
does not interpret the GPA as an L2 IPA, and does not try to populate
the nested/shadow stage-2 selected by the vCPU's last run state.

The patches are:

 - Allow callers of kvm_pgtable_get_leaf() to pass walk flags, so the
   prefault path can walk stage-2 under the MMU read lock.

 - Add arm64 support for KVM_PRE_FAULT_MEMORY.

 - Enable pre_fault_memory_test on arm64.

 - Add a backing-source option to pre_fault_memory_test.

 - Add a nested (NV) selftest that prefaults on a vCPU whose last-run
   context is backed by a shadow stage-2 MMU with an empty nested
   stage-2 root.

The prefault flag and page_size output in the stage-2 fault descriptor
remain in this series so the arm64 implementation can advance by the
mapping granule installed by the fault path and report poison without
queueing a SIGBUS.

Tested with pre_fault_memory_test under an arm64 QEMU setup with
anonymous, shmem, anonymous_thp, anonymous_hugetlb and shared_hugetlb
backings, including 64K, 2M and 32M hugetlb pools, and with the new
nv_pre_fault_memory_test on an NV-capable setup.

=== Changes since v4 [2] ===

 - Reworked nested virt semantics: arm64 now treats the ioctl GPA as the
   VM/memslot IPA and always targets the canonical stage-2. It no longer
   translates an L2 IPA through L1's stage-2.

 - Documented the arm64 nested behavior in the KVM API text.

 - Switch to the canonical stage-2 with the vCPU put/load helpers when
   the vCPU last ran with a nested/shadow MMU, keeping VMID, VNCR and
   shadow-MMU refcount state consistent.

 - Split the kvm_pgtable_get_leaf() walk-flag plumbing into a prep patch
   and walk existing mappings with KVM_PGTABLE_WALK_SHARED under the MMU
   read lock.

 - Tightened prefault fault handling: preserve fault info, set IL in the
   synthetic ESR, handle existing mappings, return -EAGAIN for invalid
   memslot races, and report -EHWPOISON without queueing SIGBUS.

 - Avoid directly walking stage-2 page tables when pKVM is enabled.
   Protected VMs remain unsupported via -EOPNOTSUPP.

 - Preserve the selected selftest memory backing when recreating the
   racing memslot.

 - Add the nested (NV) prefault selftest, including an empty nested
   stage-2 root to catch accidental L2-IPA interpretation.

=== Changes since v3 [3] ===

 - Return -EOPNOTSUPP for protected VMs.

 - Reworked nested-vCPU handling to translate an L2 IPA through L1's
   stage-2. This has been superseded by the canonical VM-IPA semantics
   described above.

 - Make page_size unsigned and keep local declarations ordered at the
   top of kvm_arch_vcpu_pre_fault_memory().

=== Changes since v2 [4] ===

 - Update the synthetic fault info. Thanks Suzuki.

 - Remove the selftest change for unaligned mmap allocations. Thanks
   Sean.

[1]: https://lore.kernel.org/kvm/20240710174031.312055-1-pbonzini@redhat.com/
[2]: https://lore.kernel.org/linux-arm-kernel/20260113152643.18858-1-jackabt.amazon@gmail.com/
[3]: https://lore.kernel.org/linux-arm-kernel/20251119154910.97716-1-jackabt.amazon@gmail.com/
[4]: https://lore.kernel.org/linux-arm-kernel/20251013151502.6679-1-jackabt.amazon@gmail.com/

Jack Thomson (5):
  KVM: arm64: Pass walk flags to kvm_pgtable_get_leaf()
  KVM: arm64: Add pre_fault_memory implementation
  KVM: selftests: Enable pre_fault_memory_test for arm64
  KVM: selftests: Add option for different backing in pre-fault tests
  KVM: selftests: Add nested pre-fault test for arm64

 Documentation/virt/kvm/api.rst                |  18 +-
 arch/arm64/include/asm/kvm_pgtable.h          |   5 +-
 arch/arm64/kvm/Kconfig                        |   1 +
 arch/arm64/kvm/arm.c                          |   1 +
 arch/arm64/kvm/hyp/nvhe/mem_protect.c         |  10 +-
 arch/arm64/kvm/hyp/pgtable.c                  |   5 +-
 arch/arm64/kvm/mmu.c                          | 164 +++++++++++++-
 arch/arm64/kvm/nested.c                       |   2 +-
 tools/testing/selftests/kvm/Makefile.kvm      |   2 +
 .../kvm/arm64/nv_pre_fault_memory_test.c      | 200 ++++++++++++++++++
 .../selftests/kvm/pre_fault_memory_test.c     | 150 ++++++++++---
 11 files changed, 513 insertions(+), 45 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/arm64/nv_pre_fault_memory_test.c


base-commit: 98f826f3c500fda08d51fca434b7aefa6a2f7076
-- 
2.43.0