On Thu, May 14, 2020 at 05:19:40PM +0900, David Stevens wrote:

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

quoted

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.

Uh, that means you actually do something and there's locking involved.
Makes stuff more complicated, invariant attributes are a lot easier
generally. Registering that uuid just always doesn't work, and blocking
when you're exporting?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch