QTMOUNT allows exportfs to detect an attempt to open a /srv file that has been opened and mounted somewhere on the system running exportfs (i'll refer to that as the `called system' and the client of exportfs as the `calling system').
exportfs opens the /srv file, mounts that file descriptor itself, and then serves 9p itself to access the resulting name space, when the caller mounts it (on the calling system), just as if the caller had been able to open the /srv file directly to mount it (on the calling system). that's needed because once a file is serving 9p, it can't be read or written directly (see the CMSG flag on chans). it isn't to do with exporting name spaces that contain remote elements.
