[PATCH 0/2] KVM: nSVM: Expose DecodeAssists to L1

[Tina Zhang]

The SVM DecodeAssists feature is reported in CPUID
Fn8000_000A_EDX[7].  When available, hardware provides the length and bytes
of the intercepted instruction in the VMCB, allowing a hypervisor to consume
the decode information directly instead of re-decoding the instruction in
software on relevant VM-Exit paths.

KVM currently does not expose DecodeAssists to nested SVM guests, even when
the host supports it, and does not propagate the hardware-provided
instruction length and bytes from VMCB02 to VMCB12 on nested VM-Exit.  This
leaves L1 with an incomplete virtual SVM CPUID model and prevents L1 from
using the same hardware-assisted decode information that KVM receives for
L2 exits.

The missing virtualization was observed in practice with Hyper-V as L1,
where the absence of DecodeAssists prevented nested SVM from being made
available to L2 guests.  The fix is not Hyper-V specific.  Complete nested
SVM virtualization of DecodeAssists by advertising the feature to L1 when
supported by hardware, and by copying the decode-assist fields into VMCB12
on nested VM-Exit.

Add a selftest that triggers a nested page fault from L2 and verifies that
L1 sees a non-zero instruction length and instruction bytes matching the
faulting instruction.

Tested with:

  make -C tools/testing/selftests TARGETS=kvm run_tests

Tina Zhang (2):
  KVM: nSVM: Virtualize DecodeAssists for nested guests
  KVM: selftests: Add nested SVM DecodeAssists test

 arch/x86/kvm/svm/nested.c                     | 47 ++++++++-
 arch/x86/kvm/svm/svm.c                        |  3 +
 arch/x86/kvm/svm/svm.h                        |  6 ++
 tools/testing/selftests/kvm/Makefile.kvm      |  1 +
 .../selftests/kvm/include/x86/processor.h     |  1 +
 .../kvm/x86/svm_nested_decode_assists_test.c  | 99 +++++++++++++++++++
 6 files changed, 156 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/kvm/x86/svm_nested_decode_assists_test.c

-- 
2.43.0