The function f(x1,x2)=x1*log(1+x2/x1) is a concave function. However, it can not be used in the context of cvx as it does not appear in DCP rule set. It is a typical function in communication systems, where x1 is the bandwidth and x2 is the transmit power.

Does any one know how I can handle this function in CVX?

This comment may help. The Shannon capacity of a communication channel with gain H and noise power spectral density N_0 is defined as C = Wlog_2 (1+\frac{|H|^2 P}{ N_0W}).