virtio: make add_buf return capacity remaining

This API change means that virtio_net can tell how much capacity
remains for buffers.  It's necessarily fuzzy, since
VIRTIO_RING_F_INDIRECT_DESC means we can fit any number of descriptors
in one, *if* we can kmalloc.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Dinesh Subhraveti <dineshs@us.ibm.com>

drivers/block/virtio_blk.c

@@ -139,7 +139,7 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
 		}
 	}
 
-	if (vblk->vq->vq_ops->add_buf(vblk->vq, vblk->sg, out, in, vbr)) {
+	if (vblk->vq->vq_ops->add_buf(vblk->vq, vblk->sg, out, in, vbr) < 0) {
 		mempool_free(vbr, vblk->pool);
 		return false;
 	}

drivers/char/hw_random/virtio-rng.c

@@ -51,7 +51,7 @@ static void register_buffer(void)
 
 	sg_init_one(&sg, random_data+data_left, RANDOM_DATA_SIZE-data_left);
 	/* There should always be room for one buffer. */
-	if (vq->vq_ops->add_buf(vq, &sg, 0, 1, random_data) != 0)
+	if (vq->vq_ops->add_buf(vq, &sg, 0, 1, random_data) < 0)
 		BUG();
 	vq->vq_ops->kick(vq);
 }

drivers/char/virtio_console.c

@@ -65,7 +65,7 @@ static int put_chars(u32 vtermno, const char *buf, int count)
 
 	/* add_buf wants a token to identify this buffer: we hand it any
 	 * non-NULL pointer, since there's only ever one buffer. */
-	if (out_vq->vq_ops->add_buf(out_vq, sg, 1, 0, (void *)1) == 0) {
+	if (out_vq->vq_ops->add_buf(out_vq, sg, 1, 0, (void *)1) >= 0) {
 		/* Tell Host to go! */
 		out_vq->vq_ops->kick(out_vq);
 		/* Chill out until it's done with the buffer. */
@@ -85,7 +85,7 @@ static void add_inbuf(void)
 	sg_init_one(sg, inbuf, PAGE_SIZE);
 
 	/* We should always be able to add one buffer to an empty queue. */
-	if (in_vq->vq_ops->add_buf(in_vq, sg, 0, 1, inbuf) != 0)
+	if (in_vq->vq_ops->add_buf(in_vq, sg, 0, 1, inbuf) < 0)
 		BUG();
 	in_vq->vq_ops->kick(in_vq);
 }

drivers/net/virtio_net.c

@@ -320,7 +320,7 @@ static bool try_fill_recv_maxbufs(struct virtnet_info *vi, gfp_t gfp)
 		skb_queue_head(&vi->recv, skb);
 
 		err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, num, skb);
-		if (err) {
+		if (err < 0) {
 			skb_unlink(skb, &vi->recv);
 			trim_pages(vi, skb);
 			kfree_skb(skb);
@@ -373,7 +373,7 @@ static bool try_fill_recv(struct virtnet_info *vi, gfp_t gfp)
 		skb_queue_head(&vi->recv, skb);
 
 		err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, 1, skb);
-		if (err) {
+		if (err < 0) {
 			skb_unlink(skb, &vi->recv);
 			kfree_skb(skb);
 			break;
@@ -527,7 +527,7 @@ static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
 	num = skb_to_sgvec(skb, sg+1, 0, skb->len) + 1;
 
 	err = vi->svq->vq_ops->add_buf(vi->svq, sg, num, 0, skb);
-	if (!err && !vi->free_in_tasklet)
+	if (err >= 0 && !vi->free_in_tasklet)
 		mod_timer(&vi->xmit_free_timer, jiffies + (HZ/10));
 
 	return err;
@@ -538,7 +538,7 @@ static void xmit_tasklet(unsigned long data)
 	struct virtnet_info *vi = (void *)data;
 
 	netif_tx_lock_bh(vi->dev);
-	if (vi->last_xmit_skb && xmit_skb(vi, vi->last_xmit_skb) == 0) {
+	if (vi->last_xmit_skb && xmit_skb(vi, vi->last_xmit_skb) >= 0) {
 		vi->svq->vq_ops->kick(vi->svq);
 		vi->last_xmit_skb = NULL;
 	}
@@ -557,7 +557,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	/* If we has a buffer left over from last time, send it now. */
 	if (unlikely(vi->last_xmit_skb) &&
-	    xmit_skb(vi, vi->last_xmit_skb) != 0)
+	    xmit_skb(vi, vi->last_xmit_skb) < 0)
 		goto stop_queue;
 
 	vi->last_xmit_skb = NULL;
@@ -565,7 +565,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	/* Put new one in send queue and do transmit */
 	if (likely(skb)) {
 		__skb_queue_head(&vi->send, skb);
-		if (xmit_skb(vi, skb) != 0) {
+		if (xmit_skb(vi, skb) < 0) {
 			vi->last_xmit_skb = skb;
 			skb = NULL;
 			goto stop_queue;
@@ -668,7 +668,7 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
 		sg_set_buf(&sg[i + 1], sg_virt(s), s->length);
 	sg_set_buf(&sg[out + in - 1], &status, sizeof(status));
 
-	BUG_ON(vi->cvq->vq_ops->add_buf(vi->cvq, sg, out, in, vi));
+	BUG_ON(vi->cvq->vq_ops->add_buf(vi->cvq, sg, out, in, vi) < 0);
 
 	vi->cvq->vq_ops->kick(vi->cvq);
 

drivers/virtio/virtio_balloon.c

@@ -84,7 +84,7 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 	init_completion(&vb->acked);
 
 	/* We should always be able to add one buffer to an empty queue. */
-	if (vq->vq_ops->add_buf(vq, &sg, 1, 0, vb) != 0)
+	if (vq->vq_ops->add_buf(vq, &sg, 1, 0, vb) < 0)
 		BUG();
 	vq->vq_ops->kick(vq);
 

drivers/virtio/virtio_ring.c

@@ -208,7 +208,11 @@ static int vring_add_buf(struct virtqueue *_vq,
 
 	pr_debug("Added buffer head %i to %p\n", head, vq);
 	END_USE(vq);
-	return 0;
+
+	/* If we're indirect, we can fit many (assuming not OOM). */
+	if (vq->indirect)
+		return vq->num_free ? vq->vring.num : 0;
+	return vq->num_free;
 }
 
 static void vring_kick(struct virtqueue *_vq)

include/linux/virtio.h

@@ -34,7 +34,7 @@ struct virtqueue {
  *	out_num: the number of sg readable by other side
  *	in_num: the number of sg which are writable (after readable ones)
  *	data: the token identifying the buffer.
- *      Returns 0 or an error.
+ *      Returns remaining capacity of queue (sg segments) or a negative error.
  * @kick: update after add_buf
  *	vq: the struct virtqueue
  *	After one or more add_buf calls, invoke this to kick the other side.

net/9p/trans_virtio.c

@@ -200,7 +200,7 @@ p9_virtio_request(struct p9_client *client, struct p9_req_t *req)
 
 	req->status = REQ_STATUS_SENT;
 
-	if (chan->vq->vq_ops->add_buf(chan->vq, chan->sg, out, in, req->tc)) {
+	if (chan->vq->vq_ops->add_buf(chan->vq, chan->sg, out, in, req->tc) < 0) {
 		P9_DPRINTK(P9_DEBUG_TRANS,
 			"9p debug: virtio rpc add_buf returned failure");
 		return -EIO;