asynchronous commands support

[igaw]

This might warrant moving to a separate issue, but has some overlap here: have you considered a v2 API that allows for asynchronous commands?

It's something we'll likely need in MI; but I assume there are similarities in in-band ioctl too. What happens when you issue a long-running command (say, Sanitize) via ioctl? is there any way to do that non-blocking?

Originally posted by @jk-ozlabs in #448

[igaw]

@jk-ozlabs we added a set of functions to handle asynchronous command execution. The implementation is fairly simple: you can submit commands via a handle and wait for all commands to complete. We deliberately kept it this simple to avoid overengineering the API in the absence of concrete use cases. I suppose it’s possible to open two handles to parallelize command execution, but this still needs to be tested.

EDIT: the async support depends on io_uring, but I just realized the io_uring is attached to the global context, so the two handle idea wont work.

[jk-ozlabs]

Thanks, I shall take a look!

[igaw]

So io_uring supports out-of-order completion, it seems. This means it should be able to submit a long-running command A and a short command B. We should be able to submit B and wait only for B. After that, we can wait for A. Though for this to work, we need to introduce a command submission handle that we can wait on. Is this what you are looking for?