I try to compute the performance of a portfolio of a CDS. I already know how to mark-to-market a CDS but typically, the first time you enter a CDS, you invest zero as the mark-to-market of a CDS is zero at t=0, so that the value of the portfolio is still zero. So when you exit the CDS, you do a non-zero P&L. But how to compute the return in this case, because in the usual definition of a return, it would imply a division by zero, right? I'm pretty confused on the subject, do you have any clue?