Sending CQE to a different ring

[Artyom Pavlov <newpavlov@gmail.com>]

Greetings!

A common approach for multi-threaded servers is to have a number of 
threads equal to a number of cores and launch a separate ring in each 
one. AFAIK currently if we want to send an event to a different ring, we 
have to write-lock this ring, create SQE, and update the index ring. 
Alternatively, we could use some kind of user-space message passing.

Such approaches are somewhat inefficient and I think it can be solved 
elegantly by updating the io_uring_sqe type to allow accepting fd of a 
ring to which CQE must be sent by kernel. It can be done by introducing 
an IOSQE_ flag and using one of currently unused padding u64s.

Such feature could be useful for load balancing and message passing 
between threads which would ride on top of io-uring, i.e. you could send 
NOP with user_data pointing to a message payload.

Best regards,
Artyom Pavlov.