OpenVMM Logging

Configuring OpenVMM logging messages to emit

To configure logging, set the OPENVMM_LOG environment variable. The default level is info. For example:

• OPENVMM_LOG=debug — enable debug events from all modules
• OPENVMM_LOG=info,mesh=trace — enable trace events from the mesh crate and info events from everything else

This is backed by the EnvFilter type; see the associated documentation for more details.

Span events

By default, OpenVMM does not log span enter/exit events. To enable them, set OPENVMM_LOG_SPANS=1.

Rate limiting

Trace events that can be triggered repeatedly by guest interactions are rate-limited by default. To disable rate limiting (useful for debugging), set OPENVMM_DISABLE_TRACING_RATELIMITS=1.

OpenTelemetry

Build OpenVMM with the otel feature, then set OPENVMM_OTEL=1 to emit enabled spans through the platform's native tracing subsystem: ETW on Windows and user_events on GNU/Linux. Other platforms do not currently have a native OpenTelemetry trace sink. For example:

cargo build -p openvmm --features otel

OPENVMM_OTEL=1 \
OTEL_RESOURCE_ATTRIBUTES="service.instance.id=boot-test-42" \
target/debug/openvmm [...]

The native events preserve OpenTelemetry trace and span identifiers. They are not sent directly to an OpenTelemetry Collector or Jaeger; a platform-specific agent must collect and forward them. Windows emits Span events from provider openvmm, and Linux emits the user_events:openvmm_L4K1 tracepoint. On Linux, the process must have write access to the tracefs user_events_data file; OpenTelemetry initialization fails if user_events is unavailable or not writable.

Configuring OpenHCL Trace Logging

OpenHCL also supports EnvFilter-style trace logging, configured via the -c OPENVMM_LOG= command line argument. The -c flag passes arguments to OpenHCL initialization. The filter syntax is the same as for OpenVMM.

OpenHCL tracing can also be configured and dumped at runtime with ohcldiag-dev. See: OpenHCL Diagnostics

To retrieve OpenHCL log output at runtime, an output console or file must attach to the OpenHCL logging COM port. By default, OpenHCL outputs to COM3.

To open a new terminal window with global OpenHCL debug level tracing enabled:

openvmm -c "OPENVMM_LOG=debug" --com3 "term,name=VTL2 OpenHCL" [...]

Configure log levels of only a given module name:

openvmm -c "OPENVMM_LOG=mesh=trace" --com3 "term,name=VTL2 OpenHCL" [...]

Multiple modules can be specified by separating them with a comma:

openvmm -c "OPENVMM_LOG=mesh=trace,nvme_driver=trace" \
    --com3 "term,name=VTL2 OpenHCL" [...]

Capturing the ETW traces on the host

On Windows, OpenVMM also logs to ETW, via the Microsoft.HvLite provider.

To capture the trace, start a session:

logman start trace <SESSION_NAME> -ow -o trace.etl `
    -p "{22bc55fe-2116-5adc-12fb-3fadfd7e360c}" 0xffffffffffffffff 0xff `
    -nb 16 16 -bs 16 -mode 0x2 -ets

To flush:

logman update <SESSION_NAME> -ets -fd

To stop:

logman stop <SESSION_NAME> -ets

To decode as CSV:

tracerpt trace.etl -y -of csv -o trace.csv -summary trace-summary.txt