I will outline a framework for the regulation of star formation in high-mass galaxies that has been motivated by X-ray observations. According to this framework, feedback regulates star formation by suspending the circumgalactic medium CGM in a state of marginal thermal instability, which links a galaxys star-formation rate and central black-hole mass with the cooling rate of the CGM. One of the key test of this framework will be the diffuse X-ray luminosity function of galaxies, which can be measured by the X-ray surveyor.