The SVM DecodeAssists feature provides decode state for selected VM-Exits. KVM currently does not expose this feature to L1. Some L1 hypervisors may therefore treat the platform's SVM support as incomplete.
In practice, this was observed with Hyper-V running on top of KVM. Hyper-V appears to require DecodeAssists before enabling nested SVM for its guests. Virtualizing the feature lets users enable Hyper-V virtualization features inside a Windows VM when needed, e.g. to run QEMU/KVM in WSL. Virtualize both parts of DecodeAssists for nested SVM. For emulated MOV CR/DR, INTn, INVLPG, and related intercepts, populate EXITINFO1 as specified by the architecture. INVLPGA's address remains in saved guest rAX. Leave EXITINFO1 unchanged when DecodeAssists is not exposed. For data #NPF and intercepted data #PF exits, propagate current hardware instruction bytes when available. For an emulator-generated #NPF, keep the bytes used to decode the instruction and fetch any missing tail. Queued #PF exits, including userspace-injected exceptions, use an on-demand fetch from the current L2 CS:RIP. Instruction-fetch faults report zero bytes, and SEV guests do not use the memory-fetch fallback. The selftest covers synthesized EXITINFO1, hardware and synthesized instruction bytes, and truncated fetches. Two MMIO cases check that emulator-generated #NPF exits retain cached bytes after instruction memory changes, while userspace-injected #PF exits do not reuse an old emulator cache after MMIO completion. The updated selftest passed with kvm.force_emulation_prefix both disabled and enabled in an isolated VM using the v6 implementation. SEV paths have not been tested. Changes since v5: - Rebase onto kvm-x86/next at the base commit listed below. - Reuse __linearize() for INVLPG and name the instruction-info field invlpg_linear_addr. - Integrate EXITINFO1 generation into svm_check_intercept() instead of using a separate switch. Leave undefined EXITINFO2 fields unchanged. - Invalidate instruction bytes by clearing only insn_len. Leave the VMCB12 fields untouched when DecodeAssists is not exposed, and stop clearing unused instruction-byte buffer tails. - Remove the extra argument to nested_svm_exit_handled(); opcode exits are excluded by the instruction-byte applicability check. - Replace the synthesized-byte staging buffer with direct VMCB02 updates, and add an emulator accessor instead of inspecting the fetch cache in nSVM. Reuse the cache only for the current emulator exception. - Add the complete fallback before cache reuse so that no intermediate patch depends on a later patch to fetch missing bytes. Reuse the generic guest-memory read helper with instruction-fetch permissions, checking address boundaries between page-sized chunks. - Extend the MOVSB regression test to modify the opcode while MMIO is pending, restore userspace-injected #PF coverage, and give the MMIO source its own page. Both MMIO cases run without forced emulation. Previous versions: v5: https://lore.kernel.org/r/[email protected] v4: https://lore.kernel.org/r/[email protected] v3: https://lore.kernel.org/r/[email protected] v2: https://lore.kernel.org/r/[email protected] v1: https://lore.kernel.org/r/[email protected] Tina Zhang (8): KVM: x86: Provide INVLPG linear address to intercept handlers KVM: nSVM: Synthesize DecodeAssists EXITINFO for emulated intercepts KVM: nSVM: Track valid hardware DecodeAssist bytes KVM: nSVM: Propagate hardware DecodeAssist bytes to VMCB12 KVM: nSVM: Fetch DecodeAssist bytes for synthesized faults KVM: nSVM: Use emulator bytes for synthesized nested #NPF KVM: nSVM: Advertise DecodeAssists to L1 KVM: selftests: Add nested SVM DecodeAssists test arch/x86/kvm/cpuid.c | 1 + arch/x86/kvm/emulate.c | 40 ++ arch/x86/kvm/kvm_emulate.h | 4 + arch/x86/kvm/svm/nested.c | 163 +++++- arch/x86/kvm/svm/svm.c | 34 +- arch/x86/kvm/svm/svm.h | 3 + arch/x86/kvm/x86.c | 23 +- arch/x86/kvm/x86.h | 3 + tools/testing/selftests/kvm/Makefile.kvm | 1 + .../selftests/kvm/include/x86/processor.h | 1 + .../kvm/x86/svm_nested_decode_assists_test.c | 547 ++++++++++++++++++ 11 files changed, 814 insertions(+), 6 deletions(-) create mode 100644 tools/testing/selftests/kvm/x86/svm_nested_decode_assists_test.c base-commit: 76671054f9a1ff6abb976583cd8da37650acdc97 -- 2.43.7
