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STATUS_RING_PREVIOUSLY_FULL
A successful removal changed producer backpressure state
STATUS_RING_PREVIOUSLY_FULL reports a successful packet removal whose pre-operation state was full. The key consequence is not the removed payload itself but the creation of free capacity. A producer that could not advance while the ring was full may now be able to submit data.
A classic shared ring distinguishes readable data from writable capacity with read and write positions. The Linux Hyper-V VMBus documentation describes two shared-memory rings per channel and notes the conventional empty test based on equal indices; its implementation keeps unused space so full and empty remain distinguishable. Microsoft KMCL similarly exposes packet send and incoming-packet callbacks around an opposite endpoint. These sources are useful models for interpreting the boundary without assuming that every occurrence originates in VMBus.
Diagnose this status as a backpressure-release event. Capture how long the ring remained full, which producer was blocked or deferred, whether the consumer published its updated read position before signalling, and whether the producer actually resumed. Re-queueing the packet that was just removed is usually the wrong reaction because the removal already succeeded. If throughput stalls, inspect the wakeup protocol and memory ordering around free-space publication.
What to inspect
- Measure full-ring duration and producer wait time.
- Log the read-index publication and subsequent notification decision.
- Check that success-style NTSTATUS handling does not route this value through an error path.
References
- Microsoft Open Specifications: NTSTATUS values
- Linux kernel: VMBus ring buffers
- Microsoft WDK: VmbPacketSend
- Microsoft WDK: incoming VMBus packets
- Microsoft: Using NTSTATUS values
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