System performance is a key factor to take into account throughout the software life cycle of modern computer systems, mostly due to their typical characteristics such as distributed deployment, code mobility, and platform heterogeneity. An open challenge in this direction is to integrate the performance validation as a transparent and efficient activity in the system development process. Several methodologies have been proposed to automate the transformation of software/hardware models into performance models. In this paper, we do not take a transformational approach; rather, we present a framework to integrate a software model with a platform model in order to build a performance model. Performance indices are obtained from simulation of the resulting performance model. Our framework provides a library of predefined resource models, model annotation and integration procedures, and simulation support that makes the performance analysis a much easier activity. We present the results obtained from two different industrial case studies that show the maturity and the stability of our approach.