Have the butcher bone the loin and tie it back on its rack of bones. Rub it with the salt, pepper, and rosemary. Wrap it loosely. Let rest at room temperature an hour or so. It is vital to the cooking time that the meat isn't refrigerator-cold. When the coals are pink and covered with white ash, lay the roast on top of the oiled grill rack directly over the drip pan. Put on the cover, with its vents fully open. The lower vents under the coals should also be fully open. Roast do not baste or peek for about one and a half hours to an internal temperature of 150 degrees. Let the pork rest on a board for about 10 minutes before carving so that it will retain all its juices.