gnttab_entry
head = gnttab_entry(head);
gnttab_free_head = gnttab_entry(head);
gnttab_entry(head) = GNTTAB_LIST_END;
*gnttab_free_tail_ptr = gnttab_entry(ret + count - 1);
gnttab_entry(ref) = gnttab_free_head;
gnttab_entry(i) = i + 1;
gnttab_entry(i) = GNTTAB_LIST_END;
gnttab_entry(gnttab_last_free) = start;
ref = gnttab_entry(head);
*private_head = gnttab_entry(g);
gnttab_entry(release) = *private_head;