According to the Thomistic view of omniscience, what is or is not possible is determined by the range of God’s power. The range of God’s power therefore determines what modal truths there are. If God has perfect self-knowledge, then God’s knowledge of the range of his own power determines what modal truths God knows.
It seems that this Thomistic view of omniscience causes trouble for the distinction between possible and feasible worlds for God. Call an infeasible world a possible world that, for whatever reason, God cannot actualize. If God cannot actualize a world, the range of his power does not extend there, and so neither does his knowledge. Therefore, either infeasible worlds are not possible worlds after all, or if they are God cannot have knowledge of them.