# Short block I/O and device error contract (K1516–K1520)

A read request owns one 512-byte destination sector plus one status byte.
The virtio used length must be exactly 513, the used head must match, and the
status must be zero before a sector may be delivered. A used length of 257 is
`ShortRead`; status 1 is `IoError`; status 2 is `Unsupported`. Unknown status
is `DeviceFailure`. These errors leave the request and DMA buffer live until
the device is reset and QueueReady is observed as zero. The caller's output
buffer must retain its previous bytes on all error paths.

The VFS-facing read result must expose I/O failure explicitly. No failed or
short transfer may be converted into a successful sector, end-of-file, or a
zero-filled sector. A later healthy read after reset must still succeed.

Local tests cover the full 513-byte success, 257-byte short response,
device I/O and unsupported statuses, unchanged ownership on error, reset,
and retry. K1520 must bind the same implementation to a guest-visible VFS
error path and retain serial/QMP evidence across direct and UEFI profiles.