Virtualization - May 2015 - [vhost] vhost_get_vq_desc, vq->iov mapping

From what I know, qemu and vhost backends use some functions in order 
to access the memory between the guest and the backend.
 From what I see, vhost does this with vhost_get_vq_desc().
How much do vq->iov mappings remain valid? Does it get unmapped at some 
time (you could answer me within vhost-net or vhost-scsi
as examples)?