Re: Sending CQE to a different ring

[Artyom Pavlov <newpavlov@gmail.com>]

>> Yes, passing positive result value would make more sense than PID of
>> submitter, which is rarely, if ever, needed. IIUC we would not be able
>> to use linking with such approach, since sqe->len has to be set in
>> user code based on a received CQE, but I guess it should be fine in
>> practice.
> 
> Right, and using sqe->len and passing it through makes a lot more sense
> in general as you can pass whatever you want there. If you want to use
> the pid, you can use it like that. Or for whatever else you'd want. That
> gives you both 'len' and 'user_data' as information you can pass between
> the rings.
> 
> It could also be used as `len` holding a message type, and `user_data`
> holding a pointer to a struct. For example.

I like IORING_OP_WAKEUP_RING with sqe->len being copied to cqe->result. 
The only question I have is how should "negative" (i.e. bigger or equal 
to 2^31) lengths be handled. They either should be copied similarly to 
positive values or we should get an error.

Also what do you think about registering ring fds? Could it be beneficial?