If, as you claim, morality is obeying god, how do you know that obeying god is good? Isn't that totally circular?

If morality is obeying god, then obeying god is morality.

If we grant the former, then the latter follows by tautological necessity.

Is it circular?

Insofar as tautologies are circular, sure. Here is another circular tautology:

If 2 + 2 = 4, then 4 = 2 + 2.

Totally circular.

Malachi 3:18; Romans 4:15, 5:13; 1 John 3:4.