vhost_net: revert upend_idx only on retriable error

Fix possible virtqueue used buffers leak and corresponding stuck
in case of temporary -EIO from sendmsg() which is produced by
tun driver while backend device is not up.

In case of no-retriable error and zcopy do not revert upend_idx
to pass packet data (that is update used_idx in corresponding
vhost_zerocopy_signal_used()) as if packet data has been
transferred successfully.

v2: set vq->heads[ubuf->desc].len equal to VHOST_DMA_DONE_LEN
in case of fake successful transmit.
Signed-off-by: Andrey Smetanin <asmetanin@yandex-team.ru>
Message-Id: <20230424204411.24888-1-asmetanin@yandex-team.ru>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Andrey Smetanin <asmetanin@yandex-team.ru>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/vhost/net.c

@@ -935,13 +935,18 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
 
 		err = sock->ops->sendmsg(sock, &msg, len);
 		if (unlikely(err < 0)) {
+			bool retry = err == -EAGAIN || err == -ENOMEM || err == -ENOBUFS;
+
 			if (zcopy_used) {
 				if (vq->heads[ubuf->desc].len == VHOST_DMA_IN_PROGRESS)
 					vhost_net_ubuf_put(ubufs);
+				if (retry)
 					nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
 						% UIO_MAXIOV;
+				else
+					vq->heads[ubuf->desc].len = VHOST_DMA_DONE_LEN;
 			}
-			if (err == -EAGAIN || err == -ENOMEM || err == -ENOBUFS) {
+			if (retry) {
 				vhost_discard_vq_desc(vq, 1);
 				vhost_net_enable_vq(net, vq);
 				break;