__gnttab_entry
#define gnttab_entry(entry) (*__gnttab_entry(entry))
if (gnttab_free_tail_ptr == __gnttab_entry(head))
last = __gnttab_entry(from);
gnttab_free_tail_ptr = __gnttab_entry(to - 1);
gnttab_free_tail_ptr = __gnttab_entry(ref);
gnttab_free_tail_ptr = __gnttab_entry(gnttab_size);