Re: [PATCH v5 0/8] KVM: nSVM: Enable DecodeAssists for nested guests

[Tina Zhang <zhang_wei@open-hieco.net>]

Hi Sean and Jim,

A gentle ping on this series. I believe v5 addresses all the comments 
from the previous review rounds. Could you please take another look when 
you have a chance? Any further comments would be appreciated.

Thanks,
Tina


On 8/24/2026 8:39 PM, Tina Zhang wrote:
> 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.  Without DecodeAssists, Hyper-V does not enable nested
> SVM because DecodeAssists is missing from KVM's virtual SVM model.
> 
> Virtualize both parts of DecodeAssists for nested SVM.  For emulated
> MOV CR/DR, INTn, INVLPG, and INVLPGA intercepts, synthesize the
> architectural EXITINFO state that hardware would provide to L1.
> 
> For data #PF and #NPF VM-Exits, provide GuestInstrBytes from fresh
> VMCB02 state, matching emulator fetch bytes, or an on-demand fetch from
> the current L2 CS:RIP, depending on how the nested VM-Exit was produced.
> Instruction-fetch faults report no instruction bytes, and encrypted
> guests do not use the on-demand fallback.
> 
> Add a focused selftest covering synthesized EXITINFO, hardware and
> synthesized GuestInstrBytes, truncated instruction fetching, and
> instruction-fetch faults.  The test has been run with
> kvm.force_emulation_prefix both disabled and enabled.
> 
> Changes since v4:
> - Drop the generic queued-exception provenance tracking for delayed and
>    synthesized #PF VM-Exits.  Fetch their instruction bytes from the
>    current L2 CS:RIP when constructing VMCB12 instead.
> - Keep matching emulator fetch bytes only for synthesized #NPF exits,
>    and use the on-demand fetch to fill any missing tail.
> - Reduce the series from nine to eight patches by removing the generic
>    x86 exception-tracking patch.
> - Trim the selftest to focused cases that cover distinct implementation
>    paths and regressions, removing redundant instruction variants,
>    overlapping boundary cases, and userspace event-state coverage.
> 
> v4:
> https://lore.kernel.org/r/cover.1787116250.git.zhang_wei@open-hieco.net
> 
> Changes since v3:
> - Rebase onto kvm-x86/next.
> - Register DecodeAssists in the CPUID 0x8000000A SVM capability
>    initializer so that common code validates its CPUID word before the
>    SVM code enables it for nested guests.
> - Make the VMCB02 instruction-byte source const and simplify the
>    synthesized-byte copy and fallback-fetch flow.
> 
> v3:
> https://lore.kernel.org/r/cover.1785411877.git.zhang_wei@open-hieco.net
> 
> Changes since v2:
> - Rebase onto kvm-x86/next.
> - Track hardware-provided instruction bytes independently of the VMCB02
>    exit code, and preserve the bytes when L0 handles an intercepted #PF
>    before reflecting it to L1.
> - Select the instruction-byte source using host-owned VMCB02 state
>    instead of control fields in guest-owned VMCB12.
> - Record whether a queued #PF VM-Exit has a matching emulator context,
>    so userspace-injected #PF exits do not consume stale emulator bytes.
> - Stop fallback instruction fetches at noncanonical addresses and at the
>    32-bit linear-address boundary.
> - Extend the selftest with regression coverage for replacing a hardware
>    #NPF with a synthesized #NPF and for userspace-injected #PF during
>    emulation, and harden its page layout and ucall handling.
> 
> v2:
> https://lore.kernel.org/r/cover.1783999988.git.zhang_wei@open-hieco.net
> 
> Changes since v1:
> - Split the implementation into seven patches to make the individual
>    pieces easier to review.
> - Add EXITINFO virtualization for emulator-generated MOV CR/DR, INTn,
>    INVLPG, and INVLPGA intercepts.
> - Limit GuestInstrBytes propagation to data #NPF and intercepted #PF
>    exits, and clear the fields for instruction-fetch and unrelated exits.
> - Provide GuestInstrBytes for KVM-synthesized data #PF/#NPF exits.  Use
>    matching emulator bytes first and fetch missing bytes from L2 RIP as a
>    fallback, while avoiding fallback reads for SEV guests.
> - Expand the selftest beyond hardware #NPF and stale-state coverage to
>    exercise hardware, synthesized, userspace-injected, instruction-fetch,
>    page-boundary, and CS-limit cases.
> 
> v1:
> https://lore.kernel.org/r/20260629125205.52394-1-zhang_wei@open-hieco.net
> 
> Tina Zhang (8):
>    KVM: x86: Add helper to provide intercept linear addresses
>    KVM: nSVM: Synthesize DecodeAssists EXITINFO for emulated intercepts
>    KVM: nSVM: Track fresh hardware DecodeAssist bytes
>    KVM: nSVM: Propagate hardware DecodeAssist bytes to VMCB12
>    KVM: nSVM: Use emulator bytes for synthesized nested #NPF
>    KVM: nSVM: Fetch missing DecodeAssist bytes for synthesized #NPF/#PF
>    KVM: nSVM: Advertise DecodeAssists to L1
>    KVM: selftests: Add nested SVM DecodeAssists test
> 
>   arch/x86/kvm/cpuid.c                          |   1 +
>   arch/x86/kvm/emulate.c                        |  29 +-
>   arch/x86/kvm/kvm_emulate.h                    |   1 +
>   arch/x86/kvm/svm/nested.c                     | 180 ++++++-
>   arch/x86/kvm/svm/svm.c                        |  56 +-
>   arch/x86/kvm/svm/svm.h                        |  17 +-
>   tools/testing/selftests/kvm/Makefile.kvm      |   1 +
>   .../selftests/kvm/include/x86/processor.h     |   1 +
>   .../kvm/x86/svm_nested_decode_assists_test.c  | 482 ++++++++++++++++++
>   9 files changed, 754 insertions(+), 14 deletions(-)
>   create mode 100644 tools/testing/selftests/kvm/x86/svm_nested_decode_assists_test.c
> 
> 
> base-commit: 1b731e5ded480bd1e5546aed35584238661ce72e