Say what you will about the great JOHN MAYER... he knows how to play the HELL out of a guitar! 

 

The former paramour of KATY PERRY - and an endless list of women in entertainment - is quick to bare his soul, and does in his cover of BEYONCE's Ryan Teddar-written smash XO. 

 

It's unclear if this is a musical overture to PERRY who is overseas on her PRISMATIC tour, or if it's a song for the woman who reportedly split 'em up...