Sorry for the duplicate reply, didn't notice this until now.

Just storing
the uuid should be doable (assuming this doesn't change during the
lifetime of the buffer), so no need for a callback.

Directly storing the uuid doesn't work that well because of
synchronization issues. The uuid needs to be shared between multiple
virtio devices with independent command streams, so to prevent races
between importing and exporting, the exporting driver can't share the
uuid with other drivers until it knows that the device has finished
registering the uuid. That requires a round trip to and then back from
the device. Using a callback allows the latency from that round trip
registration to be hidden.

-David