I have a virtual block device that is backed by a file. Works great, but now Linux V5 broke everything. Is there a migration guide to use whatever the new functions are?

That would be a result of blk-mq. It has been in the wings for an awful long time.
I haven't looked, but I'll have a small wager that there are threads on lkml.

Later: perhaps have a read of this

You're right, I migrated over, wasn't too bad but have lots of questions. First, does this mean it handles worker threads itself and I can get rid of mine? I presumed so, so I removed it. Next, what are the correct return codes for handling the function that handles the "queue_rq" items (say if I did send it to a worker, when would I "blk_mq_start_request()", before adding to worker queue or after worker starts it? Right now I did away with worker, but I still have all these open questions. Could you find docs for it?

static struct blk_mq_ops _mq_ops = {
.queue_rq = queue_rq,
};