How should estimate log in CVX?

how should estimate log in CVX?

What are you trying to do?

help cvx/log

Disciplined convex programming information:
log(X) is concave and nondecreasing in X. When used in CVX
expressions, X must be concave.

Disciplined geometric programming information:
    log(X) is typically not used in geometric programs. Technically it
    possible to do so in certain advanced cases, because monomials and
    posynomials are treated by CVX as log-affine and log-convex
    constructs, respectively. However, such usage is undocumented and
    will not be officially supported.