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Kernel Debugging (KDNET)

Kernel Debugging is available for Windows guests via KDNET over VMBus.

Enabling and Starting the Debugger

Set up KDNET on the guest and start the debugger as described on Set up KDNET network kernel debugging manually | Microsoft Learn. Setting busparams is not necessary.

If you're using a `memdiff:` disk (the default in most examples), guest state
is not persisted between runs. You'll need to configure KDNET in the guest
each time you launch — or configure it once on the base disk image before
wrapping it with `memdiff:`.

With OpenVMM and WHP as Host

Launch the VM with UEFI and networking enabled. KDNET requires --net consomme for the network transport:

# Without OpenHCL
cargo run -- --uefi --hv --net consomme \
  --vmbus-scsi id=scsi0 \
  --disk memdiff:file:path/to/windows.vhdx,on=scsi0

# With OpenHCL
cargo run -- --uefi --hv --vtl2 --net consomme \
  --igvm path/to/openhcl.igvm \
  --vmbus-scsi id=scsi0 \
  --disk memdiff:file:path/to/windows.vhdx,on=scsi0

Known Issues

  • Networking backend: KDNET currently requires --net consomme. The consomme backend creates a new network adapter in the guest on each OpenVMM restart — this is harmless and can be ignored. Using --net vmnic:<switch id> connects but hangs immediately due to an undiagnosed vmbusproxy issue.
  • Debugger reconnection: If you quit OpenVMM without shutting down the guest first, the same WinDbg instance cannot reconnect on next boot. Close and relaunch WinDbg to reconnect.
  • Synic warnings with OpenHCL: When launching an OpenHCL VM with KDNET, you may see a stream of failed to signal synic errors from virt_whp::synic for several seconds. These do not affect VM functionality and can be ignored.

Debugging workflow

Once KDNET is connected:

  1. In WinDbg, go to File → Kernel Debug → Net and enter the key and port from the guest setup.
  2. Start the VM. WinDbg should connect during Windows boot.
  3. To break into the debugger: Debug → Break (or Ctrl+Break).
KDNET debugs the VTL0 (guest) Windows kernel. For debugging the VTL2
(OpenHCL) side, see [Debugging OpenHCL](../openhcl/debugging.md) which covers
serial logs, crash dumps, and `ohcldiag-dev`.