virtio net: Allow receiving SG packets

Finally this patch lets virtio_net receive GSO packets in addition
to sending them.  This can definitely be optimised for the non-GSO
case.  For comparison the Xen approach stores one page in each skb
and uses subsequent skb's pages to construct an SG skb instead of
preallocating the maximum amount of pages per skb.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (added feature bits)

drivers/net/virtio_net.c

@@ -55,6 +55,9 @@ struct virtnet_info
 	struct tasklet_struct tasklet;
 	bool free_in_tasklet;
 
+	/* I like... big packets and I cannot lie! */
+	bool big_packets;
+
 	/* Receive & send queues. */
 	struct sk_buff_head recv;
 	struct sk_buff_head send;
@@ -89,6 +92,7 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
 			unsigned len)
 {
 	struct virtio_net_hdr *hdr = skb_vnet_hdr(skb);
+	int err;
 
 	if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
 		pr_debug("%s: short packet %i\n", dev->name, len);
@@ -96,10 +100,14 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
 		goto drop;
 	}
 	len -= sizeof(struct virtio_net_hdr);
-	BUG_ON(len > MAX_PACKET_LEN);
-
-	skb_trim(skb, len);
 
+	err = pskb_trim(skb, len);
+	if (err) {
+		pr_debug("%s: pskb_trim failed %i %d\n", dev->name, len, err);
+		dev->stats.rx_dropped++;
+		goto drop;
+	}
+	skb->truesize += skb->data_len;
 	dev->stats.rx_bytes += skb->len;
 	dev->stats.rx_packets++;
 
@@ -161,7 +169,7 @@ static void try_fill_recv(struct virtnet_info *vi)
 {
 	struct sk_buff *skb;
 	struct scatterlist sg[2+MAX_SKB_FRAGS];
-	int num, err;
+	int num, err, i;
 
 	sg_init_table(sg, 2+MAX_SKB_FRAGS);
 	for (;;) {
@@ -171,6 +179,24 @@ static void try_fill_recv(struct virtnet_info *vi)
 
 		skb_put(skb, MAX_PACKET_LEN);
 		vnet_hdr_to_sg(sg, skb);
+
+		if (vi->big_packets) {
+			for (i = 0; i < MAX_SKB_FRAGS; i++) {
+				skb_frag_t *f = &skb_shinfo(skb)->frags[i];
+				f->page = alloc_page(GFP_ATOMIC);
+				if (!f->page)
+					break;
+
+				f->page_offset = 0;
+				f->size = PAGE_SIZE;
+
+				skb->data_len += PAGE_SIZE;
+				skb->len += PAGE_SIZE;
+
+				skb_shinfo(skb)->nr_frags++;
+			}
+		}
+
 		num = skb_to_sgvec(skb, sg+1, 0, skb->len) + 1;
 		skb_queue_head(&vi->recv, skb);
 
@@ -485,6 +511,12 @@ static int virtnet_probe(struct virtio_device *vdev)
 	 * the timer. */
 	vi->free_in_tasklet = virtio_has_feature(vdev,VIRTIO_F_NOTIFY_ON_EMPTY);
 
+	/* If we can receive ANY GSO packets, we must allocate large ones. */
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4)
+	    || virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6)
+	    || virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_ECN))
+		vi->big_packets = true;
+
 	/* We expect two virtqueues, receive then send. */
 	vi->rvq = vdev->config->find_vq(vdev, 0, skb_recv_done);
 	if (IS_ERR(vi->rvq)) {
@@ -571,7 +603,9 @@ static unsigned int features[] = {
 	VIRTIO_NET_F_CSUM, VIRTIO_NET_F_GUEST_CSUM,
 	VIRTIO_NET_F_GSO, VIRTIO_NET_F_MAC,
 	VIRTIO_NET_F_HOST_TSO4, VIRTIO_NET_F_HOST_UFO, VIRTIO_NET_F_HOST_TSO6,
-	VIRTIO_NET_F_HOST_ECN, VIRTIO_F_NOTIFY_ON_EMPTY,
+	VIRTIO_NET_F_HOST_ECN, VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
+	VIRTIO_NET_F_GUEST_ECN, /* We don't yet handle UFO input. */
+	VIRTIO_F_NOTIFY_ON_EMPTY,
 };
 
 static struct virtio_driver virtio_net = {