# Virtio queue backpressure contract (K1496–K1500)

The queue owns a bounded descriptor budget. A request claims a nonzero number
of descriptors with one live head and generation. If the remaining budget is
too small, submission returns `Again` (EAGAIN) and leaves every existing
claim and counter unchanged. No descriptor or avail index is published for a
rejected request.

A completion frees only the descriptors of its live head. The caller may then
retry a rejected request and observe progress. An unknown or repeated head is
rejected without changing the budget. Reset may retire outstanding claims
only after device status and queue readiness are observed as zero; before
that, DMA remains owned or quarantined. Local tests cover capacity, one extra
request, retry, reset, and a removed-capacity-guard mutant. K1500 must prove
the request ordering with a real QEMU queue in both boot paths.