How to write:r<=x*log(1+k/x) in CVX

How to write:r<=x*log(1+k/x),where k>0 and k is a constant .And in this constraint,x and r are all variables >= 0 .thanks.

r <= -rel_entr(x,x+k)

Thank you very much for your answer, it solved my difficulties very promptly. Since I am not very familiar with CVX forum, I may not be able to search with suitable keywords…I’m sorry to trouble you.