[PATCH 0/8] KVM: arm64: Add KVM_PRE_FAULT_MEMORY support

[Lorenzo Stoakes (ARM)]

This series implements the KVM stage 2 page table pre-faulting feature for
arm64.

== Foundations ==

The series begins by establishing required foundations:

1. Updating kvm_s2_fault_desc to independently store the exception syndrome
   register (ESR) value, and updating all code paths to use this value
   exclusively.

This is needed so we can later generate a synthetic fault to perform the
pre-faulting - we need to be sure the code doesn't grab an incorrect ESR
from elsewhere.

2. Updating kvm_s2_fault_desc to independently store the kvm_s2_mmu and
   updating all code paths to use this value exclusively.

Similarly this is needed so we can generate a synthetic fault against the
canonical stage 2 MMU (it would make no sense for it to touch nested shadow
page tables) - we need to be sure that the code doesn't grab an incorrect
MMU from elsewhere.

3. Updating the abort paths which consume kvm_s2_fault_desc to also return
   a kvm_s2_fault_result data structure.

To perform pre-faulting the code must know the granule size of what was
just walked. So the abort paths have to tell us what that was.

4. Pass walk flags to kvm_pgtable_get_leaf() to permit walking page tables
   under the MMU read lock.

This is Jack's patch, verbatim, which allows the use of the
KVM_PGTABLE_WALK_SHARED flag to walk page tables under the MMU read lock.

Pre-faulting requires it to be able to work in parallel as specified by the
API. The read lock precludes page tables being torn down behind our back.

== Implementation ==

Pre-faulting is implemented in kvm_arch_vcpu_pre_fault_memory() whose job
is to pre-fault the stage 2 page tables which map a specific GPA (which,
for arm64, is the guest's IPA).

This function is called by kvm_vcpu_pre_fault_memory() for each GPA in the
range, which itself is ultimately invoked by userland via the
KVM_PRE_FAULT_MEMORY ioctl.

The implementation is simple - try to walk to the stage 2 page table
mapping the GPA - if unmapped, fault it in through a synthetic page fault.

pKVM is not supported regardless of whether the VM is protected or
not.

This is because pKVM instantiates vCPUs upon run, but pre-faulting is
typically performed before a vCPU is run. It would be confusing and
inconsistent to error out on non-running vCPUs but to pre-fault running
ones.

Jack's original test suite is also included in the series.

== Credits ==

This series is based, with gratitude, on Jack Thomson's series and their
respins (links provided below) as well as the feedback he received.

The series includes Jack's v5 "KVM: arm64: Pass walk flags to
kvm_pgtable_get_leaf()" patch verbatim, and all three of his test patches,
two of which required minor fixups.

Link: https://patch.msgid.link/20260612162354.73378-1-jackabt.amazon@xxxxxxxxx/
Link: https://patch.msgid.link/20260113152643.18858-1-jackabt.amazon@xxxxxxxxx/
Link: https://patch.msgid.link/20251119154910.97716-1-jackabt.amazon@xxxxxxxxx/
Link: https://patch.msgid.link/20251013151502.6679-1-jackabt.amazon@xxxxxxxxx/
Link: https://patch.msgid.link/20250911134648.58945-1-jackabt.amazon@xxxxxxxxx/

== Reviewer Notes ==

I synced with maintainers on this who asked me to take a look, as there hadn't
been progress on the series for some time.

Am happy to rebase again after -rc1.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
Jack Thomson (4):
      KVM: arm64: Pass walk flags to kvm_pgtable_get_leaf()
      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

Lorenzo Stoakes (ARM) (4):
      KVM: arm64: Propagate and use esr in s2fd when handling guest aborts
      KVM: arm64: Propagate and use mmu in s2fd when handling guest aborts
      KVM: arm64: Propagate and use kvm_s2_fault_result on S2 fault
      KVM: arm64: Implement KVM_PRE_FAULT_MEMORY

 Documentation/virt/kvm/api.rst                     |  17 +-
 arch/arm64/include/asm/esr.h                       | 129 +++++---
 arch/arm64/include/asm/kvm_emulate.h               |  52 +---
 arch/arm64/include/asm/kvm_pgtable.h               |   5 +-
 arch/arm64/include/asm/kvm_pkvm.h                  |   2 +-
 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                               | 324 +++++++++++++++++----
 arch/arm64/kvm/nested.c                            |   2 +-
 tools/testing/selftests/kvm/Makefile.kvm           |   2 +
 .../selftests/kvm/arm64/nv_pre_fault_memory_test.c | 206 +++++++++++++
 .../testing/selftests/kvm/pre_fault_memory_test.c  | 152 ++++++++--
 14 files changed, 736 insertions(+), 172 deletions(-)
---
base-commit: aa8e5dc6a7a2a1141ab40706a51010adcd0e57d2
change-id: 20260815-kvm-arm-prefault-9bb411b6897d

Best regards,
-- 
Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>