[$] Zero-copy I/O for ublk, three different ways

The ublk subsystem enables the creation ofuser-space block drivers that communicate with the kernel using io_uring. Drivers implemented this way showsome promise with regard to performance, but there is a bottleneck in theway: copying data between the kernel and the user-space driver's addressspace. It is thus not surprising that there is interest in implementingzero-copy I/O for ublk. The mailing lists have recently seen threedifferent proposals for how this could be done.