xen-blkfront: don't disable cache flushes when they fail

blkfront always had a robust negotiation protocol for detecting a write
cache.  Stop simply disabling cache flushes in the block layer as the
flags handling is moving to the atomic queue limits API that needs
user context to freeze the queue for that.  Instead handle the case
of the feature flags cleared inside of blkfront.  This removes old
debug code to check for such a mismatch which was previously impossible
to hit, including the check for passthrough requests that blkfront
never used to start with.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20240617060532.127975-2-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkfront.c

@@ -788,6 +788,11 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 			 * A barrier request a superset of FUA, so we can
 			 * implement it the same way.  (It's also a FLUSH+FUA,
 			 * since it is guaranteed ordered WRT previous writes.)
+			 *
+			 * Note that can end up here with a FUA write and the
+			 * flags cleared.  This happens when the flag was
+			 * run-time disabled after a failing I/O, and we'll
+			 * simplify submit it as a normal write.
 			 */
 			if (info->feature_flush && info->feature_fua)
 				ring_req->operation =
@@ -795,8 +800,6 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 			else if (info->feature_flush)
 				ring_req->operation =
 					BLKIF_OP_FLUSH_DISKCACHE;
-			else
-				ring_req->operation = 0;
 		}
 		ring_req->u.rw.nr_segments = num_grant;
 		if (unlikely(require_extra_req)) {
@@ -887,16 +890,6 @@ static inline void flush_requests(struct blkfront_ring_info *rinfo)
 		notify_remote_via_irq(rinfo->irq);
 }
 
-static inline bool blkif_request_flush_invalid(struct request *req,
-					       struct blkfront_info *info)
-{
-	return (blk_rq_is_passthrough(req) ||
-		((req_op(req) == REQ_OP_FLUSH) &&
-		 !info->feature_flush) ||
-		((req->cmd_flags & REQ_FUA) &&
-		 !info->feature_fua));
-}
-
 static blk_status_t blkif_queue_rq(struct blk_mq_hw_ctx *hctx,
 			  const struct blk_mq_queue_data *qd)
 {
@@ -908,12 +901,22 @@ static blk_status_t blkif_queue_rq(struct blk_mq_hw_ctx *hctx,
 	rinfo = get_rinfo(info, qid);
 	blk_mq_start_request(qd->rq);
 	spin_lock_irqsave(&rinfo->ring_lock, flags);
-	if (RING_FULL(&rinfo->ring))
-		goto out_busy;
 
-	if (blkif_request_flush_invalid(qd->rq, rinfo->dev_info))
-		goto out_err;
+	/*
+	 * Check if the backend actually supports flushes.
+	 *
+	 * While the block layer won't send us flushes if we don't claim to
+	 * support them, the Xen protocol allows the backend to revoke support
+	 * at any time.  That is of course a really bad idea and dangerous, but
+	 * has been allowed for 10+ years.  In that case we simply clear the
+	 * flags, and directly return here for an empty flush and ignore the
+	 * FUA flag later on.
+	 */
+	if (unlikely(req_op(qd->rq) == REQ_OP_FLUSH && !info->feature_flush))
+		goto complete;
 
+	if (RING_FULL(&rinfo->ring))
+		goto out_busy;
 	if (blkif_queue_request(qd->rq, rinfo))
 		goto out_busy;
 
@@ -921,14 +924,14 @@ static blk_status_t blkif_queue_rq(struct blk_mq_hw_ctx *hctx,
 	spin_unlock_irqrestore(&rinfo->ring_lock, flags);
 	return BLK_STS_OK;
 
-out_err:
-	spin_unlock_irqrestore(&rinfo->ring_lock, flags);
-	return BLK_STS_IOERR;
-
 out_busy:
 	blk_mq_stop_hw_queue(hctx);
 	spin_unlock_irqrestore(&rinfo->ring_lock, flags);
 	return BLK_STS_DEV_RESOURCE;
+complete:
+	spin_unlock_irqrestore(&rinfo->ring_lock, flags);
+	blk_mq_end_request(qd->rq, BLK_STS_OK);
+	return BLK_STS_OK;
 }
 
 static void blkif_complete_rq(struct request *rq)
@@ -1627,7 +1630,6 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 					blkif_req(req)->error = BLK_STS_OK;
 				info->feature_fua = 0;
 				info->feature_flush = 0;
-				xlvbd_flush(info);
 			}
 			fallthrough;
 		case BLKIF_OP_READ: