[PATCH v2 00/13] 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.

== 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 two of his test patches (with
minor fixups), plus a nested pre-fault test based on his.

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.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
v2:
* Split series - Added 1/13 for the esr.h helpers, 2/13 for converting
  callers to use them, and move what was 1/8 to 3/13 to propagate
  s2fd->esr, as per Marc.
* Made esr helpers __always_inline as per Marc.
* Renamed esr_trap_get_class() -> esr_get_ec() + dropped churn as per Marc.
* Updated 5/13 (was 3/8) to drop the kvm_s2_fault_result.mapped flag.
* Also updated 5/13 so abort handlers return -EAGAIN instead of swallowing, as
  per Marc.
* Split implementation patch further -> topup_mmu_memcache() change to 6/13,
  EHWPOISON change -> 7/13 and docs change -> 10/13, as per Marc.
* No longer mkyoung existing walked ranges, updated docs to reflect, as per
  Marc.
* Eliminated s2fd->pre_fault as per Marc.
* Eliminated the redundant page table level in pre_fault_s2() as per Marc.
* Put the commit message for 9/13 on a diet as per Marc.
* Fixed up commit message for 11/13 (was 6/8) to correct reasoning about
  exit-reason assert, as per Fuad.
* Simplified: nested test 13/13 (was 8/8), dropping the empty nested-S2
  setup and using test_supports_el2() to handle NV=0 opt-out, as per
  Wei-Lin and Fuad.
* Re-authored patch 13/13 to me as the changes are substantial enough to
  require it. Updated commit message to give Jack credit for original.

v1:
https://lore.kernel.org/r/20260825-kvm-arm-prefault-v1-0-befe8947702e@xxxxxxxxxx

---
Jack Thomson (3):
      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

Lorenzo Stoakes (ARM) (10):
      arm64: Add ESR fault helpers
      KVM: arm64: Use ESR helpers in guest abort handling
      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: Size the stage-2 memcache from the fault MMU
      KVM: arm64: Propagate EHWPOISON in kvm_s2_fault_pin_pfn()
      KVM: arm64: Implement KVM_PRE_FAULT_MEMORY
      Documentation: KVM: document arm64 KVM_PRE_FAULT_MEMORY
      KVM: selftests: Add nested pre-fault test for arm64

 Documentation/virt/kvm/api.rst                     |  18 +-
 arch/arm64/include/asm/esr.h                       |  44 +++
 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                               | 300 +++++++++++++++++----
 arch/arm64/kvm/nested.c                            |   2 +-
 tools/testing/selftests/kvm/Makefile.kvm           |   2 +
 .../selftests/kvm/arm64/nv_pre_fault_memory_test.c | 158 +++++++++++
 .../testing/selftests/kvm/pre_fault_memory_test.c  | 152 +++++++++--
 14 files changed, 615 insertions(+), 137 deletions(-)
---
base-commit: aa8e5dc6a7a2a1141ab40706a51010adcd0e57d2
change-id: 20260815-kvm-arm-prefault-9bb411b6897d

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