virtio-blk: base queue-depth on virtqueue ringsize or module param

Venkatash spake thus:

virtio-blk set the default queue depth to 64 requests, which was
insufficient for high-IOPS devices. Instead set the blk-queue depth to
the device's virtqueue depth divided by two (each I/O requires at least
two VQ entries).

But behold, Ted added a module parameter:

Also allow the queue depth to be something which can be set at module
load time or via a kernel boot-time parameter, for
testing/benchmarking purposes.

And I rewrote it substantially, mainly to take
VIRTIO_RING_F_INDIRECT_DESC into account.

As QEMU sets the vq size for PCI to 128, Venkatash's patch wouldn't
have made a change. This version does (since QEMU also offers
VIRTIO_RING_F_INDIRECT_DESC.

Inspired-by: "Theodore Ts'o" <[e-mail]>
Based-on-the-true-story-of: Venkatesh Srinivas <[e-mail]>
Cc: "Michael S. Tsirkin" <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: Frank Swiderski <[e-mail]>
Signed-off-by: Rusty Russell <[e-mail]>

drivers/block/virtio_blk.c

···
 static struct blk_mq_reg virtio_mq_reg = {
 	.ops		= &virtio_mq_ops,
 	.nr_hw_queues	= 1,
-	.queue_depth	= 64,
+	.queue_depth	= 0, /* Set in virtblk_probe */
 	.numa_node	= NUMA_NO_NODE,
 	.flags		= BLK_MQ_F_SHOULD_MERGE,
 };
+module_param_named(queue_depth, virtio_mq_reg.queue_depth, uint, 0444);
 
 static void virtblk_init_vbr(void *data, struct blk_mq_hw_ctx *hctx,
 			     struct request *rq, unsigned int nr)
···
 		goto out_free_vq;
 	}
 
+	/* Default queue sizing is to fill the ring. */
+	if (!virtio_mq_reg.queue_depth) {
+		virtio_mq_reg.queue_depth = vblk->vq->num_free;
+		/* ... but without indirect descs, we use 2 descs per req */
+		if (!virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC))
+			virtio_mq_reg.queue_depth /= 2;
+	}
 	virtio_mq_reg.cmd_size =
 		sizeof(struct virtblk_req) +
 		sizeof(struct scatterlist) * sg_elems;