Preheat the oven to 325 degrees F (165 degrees C).

Combine sage, garlic, salt, and black pepper in a small bowl; rub thoroughly all over pork loin. Place pork in a roasting pan.

Bake in the preheated oven on the center rack, uncovered, until an instant-read thermometer inserted into center reads at least 145 degrees F (63 degrees C), depending upon your desired doneness, about 3 hours.

Meanwhile, combine sugar, vinegar, water, soy sauce, and cornstarch in a small saucepan over medium heat; cook, stirring occasionally, until glaze begins to bubble and thicken slightly. Remove from heat; cover to keep warm.

Brush pork loin with glaze 3 or 4 times during the last 30 minutes of cooking. Pour remaining glaze over roast to serve.