Re: [PATCH v3 1/4] dma-buf: add support for virtio exported objects

[Daniel Vetter]

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

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