Volume by square cross sections

Hey all, Calculus homework has me a little stumped. The problem reads:

The base of a solid is the region inside the circle $\displaystyle x^2 + y^2 = 9$. Find the volume of the solid if every cross section by a plane perpendicular to the x-axis is a square.

I have a graph of the circle drawn up, as well as made the equation into $\displaystyle y = \sqrt{9 - x^2}$; however, I am not sure what to set up as far as an integral goes. A little push would be greatly appreciated.