KVM: selftests: Stress save+restore and #PF (ft. nested)

Add a stress test for save+restore while the guest is triggering and
handling #PFs, in both L1 and L2. The goal was to create a generic
selftest that would catch bugs like the one fixed by commit 5c247d08bc81
("KVM: nSVM: Use vcpu->arch.cr2 when updating vmcb12 on nested
#VMEXIT"), instead of relying on high-level testing (e.g. building GCC
in L2) to catch it.

The test tries to be as generic as possible by triggering #PFs in a
guest and installing a proper #PF handler, while the host is
continuously doing save+restore cycles. Exiting to userspace is randomly
triggered by a second thread that constantly signals the vCPU thread.

Patches (1-6) are prep patches, fixing GPR switching for nSVM and
generalizing it to cover nVMX, which is needed for the test to run
properly with nVMX. Patch 4 removes HORRIFIC_L2_UCALL_CLOBBER_HACK, as
it is no longer needed. While this series does not have the "complete"
fix added by commit 6783ca4105a7 ("KVM: selftests: Add a shameful hack
to preserve/clobber GPRs across ucall"), it's a good step in the right
direction.

Patches (7-10) add the actual test. The test is first introduced as a
simple (read: dummy) stress test that just explicitly syncs to userspace
after each #PF handling to do save+restore, then gradually evolves to
add the random signaling and nested support. After the last patch, the
test reliably reproduces the CR2 bug.

v2 -> v3:
- Rebased on top of L2 stack rework in selftests.
- Fix GPR array size (i.e. NR_GUEST_REGS) [Sashiko].
- Handle evmcs_vmlaunch() and evmcs_vmresume() [Sashiko].
- Fix off-by-one assertion error in intermediate patches.
- Increase inter-signal delay from 100us to 1msec.

v1 -> v2:
- Switch guest_regs to an array, which simplifies the offsets
  calculation and forgoes the dependency on using OFFSET() or defining
  the struct offsets for assembly otherwise.
- Move page table mapping to the test (instead of a generic helper), as
  the helper mistakenly tried to map the entire memslot, not just page
  tables.
- Do not use identity mappings for page tables as it collisions with
  GVAs used for ELF in some cases.
- Simplify page table walking by using loops.
- Make sure the signals are ignored before creating the signaling
  thread [Sashiko]
- Assert that the guest actually ran and had page faults [Sashiko]
- Add a patch to fix RAX and RFLAGS offsets in run_guest() [Sashiko]
- Initialize exception_has_payload when injecting a #UD [Sashiko]
- Only check KVM_STATE_NESTED_GUEST_MODE when running in nested mode
  [Sashiko]

v1: https://lore.kernel.org/all/20260518202514.2037078-1-yosr...
v2: https://lore.kernel.org/kvm/20260604203546.365658-1-yosry...

Yosry Ahmed (10):
  KVM: selftests: Move STR() and XSTR() definitions to test_util.h
  KVM: selftests: Fix RAX and RFLAGS VMCB offsets when running L2
  KVM: selftests: Use an array for guest_regs (and fix offsets)
  KVM: selftests: Move GPR load/save definitions outside of nSVM code
  KVM: selftests: Reuse GPR switching logic for nVMX
  KVM: selftests: Drop HORRIFIC_L2_UCALL_CLOBBER_HACK
  KVM: selftests: Add basic stress test for save+restore and #PF
    handling
  KVM: selftests: Trigger save+restore randomly in the #PF stress test
  KVM: selftests: Support running stress save+restore and #PF test in L2
  KVM: selftests: Trigger L2->L1 exits stress save+restore and #PF test

 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../testing/selftests/kvm/include/test_util.h |   3 +
 .../testing/selftests/kvm/include/x86/evmcs.h |  40 +--
 .../selftests/kvm/include/x86/processor.h     |  55 ++-
 tools/testing/selftests/kvm/include/x86/vmx.h |  63 ++--
 .../testing/selftests/kvm/lib/x86/processor.c |   2 +
 tools/testing/selftests/kvm/lib/x86/svm.c     |  60 ++--
 tools/testing/selftests/kvm/lib/x86/ucall.c   |  32 +-
 .../kvm/x86/evmcs_smm_controls_test.c         |   3 -
 tools/testing/selftests/kvm/x86/smm_test.c    |   3 -
 .../kvm/x86/stress_save_restore_pf_test.c     | 326 ++++++++++++++++++
 11 files changed, 441 insertions(+), 147 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/x86/stress_save_restore_pf_test.c


base-commit: 50406d35f5635e1cc523e61409d57e851b5f5df8
-- 
2.55.0.rc0.799.gd6f94ed593-goog