[$] BPF comes to io_uring at last

The kernel's asynchronousio_uring interface maintains two shared ring buffers:a submission queue for sending requests to the kernel, and a completion queuecontaining the results of those requests. Even with shared memory removing muchof the overhead of communicating with user space, there is still some overheadwhenever the kernel must switch to user space to give it the opportunity toprocess completion requests andqueue up any subsequent work items. Apatch set from Pavel Begunkov minimizes this overhead by lettingprogrammers extend the io_uring event loop with a BPF program that can enqueueadditional work in response to completion events. The patch set hasbeen in development for a long time, but hasfinally been accepted.