vsock/virtio: a better comment on credit update

The comment we have is just repeating what the code does.
Include the *reason* for the condition instead.

Cc: Stefano Garzarella <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Reviewed-by: Stefano Garzarella <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/vmw_vsock/virtio_transport_common.c

···
 
 	spin_unlock_bh(&vvs->rx_lock);
 
-	/* We send a credit update only when the space available seen
-	 * by the transmitter is less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE
+	/* To reduce the number of credit update messages,
+	 * don't update credits as long as lots of space is available.
+	 * Note: the limit chosen here is arbitrary. Setting the limit
+	 * too high causes extra messages. Too low causes transmitter
+	 * stalls. As stalls are in theory more expensive than extra
+	 * messages, we set the limit to a high value. TODO: experiment
+	 * with different values.
 	 */
 	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE) {
 		virtio_transport_send_credit_update(vsk,