Bring the 8 cups of stock to a boil in a large saucepan. Whisk in the polenta, pouring it into the stock in a thin stream. Cook the polenta over low heat, whisking constantly until it begins to thicken, about 5 minutes. Continue cooking over low heat, stirring frequently with a wooden spoon, until the polenta is very thick and no longer gritty, about 30 minutes. Stir in the butter and parmesan cheese, season the polenta with salt. Stir in the parsley and season with salt.