[RFC PATCH v2 09/13] virtio/vsock: implement fetch of record length

[Arseny Krasnov <arseny.krasnov@xxxxxxxxxxxxx>]

This adds transport callback which tries to fetch record begin marker from
socket's rx queue. It is called from af_vsock.c before reading data
packets of record.

Signed-off-by: Arseny Krasnov <arseny.krasnov@xxxxxxxxxxxxx>
---
 include/linux/virtio_vsock.h            |  1 +
 net/vmw_vsock/virtio_transport_common.c | 33 +++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index 7f0ef5204e33..af8705ea8b95 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -84,6 +84,7 @@ virtio_transport_dgram_dequeue(struct vsock_sock *vsk,
 			       struct msghdr *msg,
 			       size_t len, int flags);
 
+size_t virtio_transport_seqpacket_seq_get_len(struct vsock_sock *vsk);
 s64 virtio_transport_stream_has_data(struct vsock_sock *vsk);
 s64 virtio_transport_stream_has_space(struct vsock_sock *vsk);
 
diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 4328f653a477..fe1272e74517 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -420,6 +420,39 @@ static size_t virtio_transport_drop_until_seq_begin(struct virtio_vsock_sock *vv
 	return bytes_dropped;
 }
 
+size_t virtio_transport_seqpacket_seq_get_len(struct vsock_sock *vsk)
+{
+	struct virtio_vsock_sock *vvs = vsk->trans;
+	struct virtio_vsock_pkt *pkt;
+	size_t bytes_dropped;
+
+	spin_lock_bh(&vvs->rx_lock);
+
+	/* Fetch all orphaned 'RW', packets, and
+	 * send credit update.
+	 */
+	bytes_dropped = virtio_transport_drop_until_seq_begin(vvs);
+
+	if (list_empty(&vvs->rx_queue))
+		goto out;
+
+	pkt = list_first_entry(&vvs->rx_queue, struct virtio_vsock_pkt, list);
+
+	vvs->user_read_copied = 0;
+	vvs->user_read_seq_len = le32_to_cpu(pkt->hdr.flags);
+	virtio_transport_del_n_free_pkt(pkt);
+out:
+	spin_unlock_bh(&vvs->rx_lock);
+
+	if (bytes_dropped)
+		virtio_transport_send_credit_update(vsk,
+						    VIRTIO_VSOCK_TYPE_SEQPACKET,
+						    NULL);
+
+	return vvs->user_read_seq_len;
+}
+EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_seq_get_len);
+
 static ssize_t virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
 						     struct msghdr *msg,
 						     size_t user_buf_len)
-- 
2.25.1