[PATCH 0/2] KVM: x86: Fix lost nested APF VM-Exits

[Loc Nguyen <loc@ryzome.com>]

A nested asynchronous page fault delivered to L1 as a synthetic #PF 
VM-Exit can be lost after KVM queues the exception.  Most callers of
kvm_queue_exception_vmexit() arrive through kvm_multiple_exception(),
which requests event processing, but the nested APF path calls the 
helper directly.

Without KVM_REQ_EVENT, vcpu_enter_guest() can skip
kvm_check_and_inject_events() and re-enter L2.  A later VM-Exit can then
clear the pending exception while reconstructing vectoring state.  The 
APF reason remains PAGE_NOT_PRESENT, allowing a later regular #PF to
consume the stale reason.

Patch 1 requests event processing in kvm_queue_exception_vmexit() so
that all queued exception VM-Exits are processed before re-entering the 
guest.

Patch 2 adds a regression test that holds an L2 backing page missing
with userfaultfd.  The test verifies that L1 receives the synthetic #PF 
VM-Exit with the expected APF token and PAGE_NOT_PRESENT reason.

Testing was performed in a nested VMX environment:

  - x86_64 kernel build with KVM_WERROR=y
  - nested_apf_event_test fails on the base kernel with a lost nested
    APF event and passes with the fix 
  - full KVM selftest suite passes
  - kvm-unit-tests VMX suite passes with TIMEOUT=900

Loc Nguyen (2):
  KVM: x86: Request event processing for exception VM-Exits
  KVM: selftests: Add a test for lost nested APF VM-Exits

 arch/x86/kvm/x86.c                            |   2 +
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../selftests/kvm/x86/nested_apf_event_test.c | 306 ++++++++++++++++++
 3 files changed, 309 insertions(+)
 create mode 100644 tools/testing/selftests/kvm/x86/nested_apf_event_test.c


base-commit: b378201ccd5280d0fff89bbe55e1eb00620ec0d5
-- 
2.43.0