[RFC PATCH v5 02/17] vhost/vsock: read data from non-linear skb

[Arseniy Krasnov <AVKrasnov@sberdevices.ru>]

This adds copying to guest's virtio buffers from non-linear skbs. Such
skbs are created by protocol layer when MSG_ZEROCOPY flags is used. It
replaces call of 'copy_to_iter()' to 'skb_copy_datagram_iter()'- second
function can read data from non-linear skb. Also this patch uses field
'frag_off' from skb control block. This field shows current offset to
read data from skb which could be both linear or not.

Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
---
 Changelog:
 v4 -> v5:
  * Use local variable for 'frag_off'.
  * Update commit message by adding some details about 'frag_off' field.
  * R-b from Bobby Eshleman removed due to patch update.

 drivers/vhost/vsock.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 6578db78f0ae..cb00e0e059e4 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -114,6 +114,7 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 		struct sk_buff *skb;
 		unsigned out, in;
 		size_t nbytes;
+		u32 frag_off;
 		int head;
 
 		skb = virtio_vsock_skb_dequeue(&vsock->send_pkt_queue);
@@ -156,7 +157,8 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 		}
 
 		iov_iter_init(&iov_iter, ITER_DEST, &vq->iov[out], in, iov_len);
-		payload_len = skb->len;
+		frag_off = VIRTIO_VSOCK_SKB_CB(skb)->frag_off;
+		payload_len = skb->len - frag_off;
 		hdr = virtio_vsock_hdr(skb);
 
 		/* If the packet is greater than the space available in the
@@ -197,8 +199,10 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 			break;
 		}
 
-		nbytes = copy_to_iter(skb->data, payload_len, &iov_iter);
-		if (nbytes != payload_len) {
+		if (skb_copy_datagram_iter(skb,
+					   frag_off,
+					   &iov_iter,
+					   payload_len)) {
 			kfree_skb(skb);
 			vq_err(vq, "Faulted on copying pkt buf\n");
 			break;
@@ -212,13 +216,13 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 		vhost_add_used(vq, head, sizeof(*hdr) + payload_len);
 		added = true;
 
-		skb_pull(skb, payload_len);
+		VIRTIO_VSOCK_SKB_CB(skb)->frag_off += payload_len;
 		total_len += payload_len;
 
 		/* If we didn't send all the payload we can requeue the packet
 		 * to send it with the next available buffer.
 		 */
-		if (skb->len > 0) {
+		if (VIRTIO_VSOCK_SKB_CB(skb)->frag_off < skb->len) {
 			hdr->flags |= cpu_to_le32(flags_to_restore);
 
 			/* We are queueing the same skb to handle
-- 
2.25.1