nbd: add support for rotational devices

The NBD protocol defines the flag NBD_FLAG_ROTATIONAL to flag that the
export in use should be treated as a rotational device.

Add support for that flag to the kernel driver.
Signed-off-by: Wouter Verhelst <w@uter.be>
Reviewed-by: Eric Blake <eblake@redhat.com>
Link: https://lore.kernel.org/r/20240725164536.1275851-1-w@uter.beSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -350,6 +350,9 @@ static int __nbd_set_size(struct nbd_device *nbd, loff_t bytesize,
 		lim.features |= BLK_FEAT_WRITE_CACHE;
 		lim.features &= ~BLK_FEAT_FUA;
 	}
+	if (nbd->config->flags & NBD_FLAG_ROTATIONAL)
+		lim.features |= BLK_FEAT_ROTATIONAL;
+
 	lim.logical_block_size = blksize;
 	lim.physical_block_size = blksize;
 	error = queue_limits_commit_update(nbd->disk->queue, &lim);

include/uapi/linux/nbd.h

@@ -51,8 +51,9 @@ enum {
 #define NBD_FLAG_READ_ONLY	(1 << 1) /* device is read-only */
 #define NBD_FLAG_SEND_FLUSH	(1 << 2) /* can flush writeback cache */
 #define NBD_FLAG_SEND_FUA	(1 << 3) /* send FUA (forced unit access) */
-/* there is a gap here to match userspace */
+#define NBD_FLAG_ROTATIONAL	(1 << 4) /* device is rotational */
 #define NBD_FLAG_SEND_TRIM	(1 << 5) /* send trim/discard */
+/* there is a gap here to match userspace */
 #define NBD_FLAG_CAN_MULTI_CONN	(1 << 8)	/* Server supports multiple connections per export. */
 
 /* values for cmd flags in the upper 16 bits of request type */