With the development of VLSI (very large scale integrated circuit) technology, a single chip can contain over one billion transistor. Multithreading technique is the developing trend of high performance processor in the future. How to design hardware/software interface has become a major problem for multithreading processor. According to simultaneous multithreading processor requirement and the original Godson-2 architecture, the hardware/software interface for Godson-2 simultaneous multithreading processor is advanced and defined. Based on this interface, the corresponding Linux for Godson-2 SMT (simultaneous multithreading) processor is designed and implemented on Linux 2.4.20. The SPEC CPU2000 and some other benchmark programs are used to compare the performance of Godson-2 SMT processor with the superscalar processor. The results show that the Godson-2 SMT processor designed with the interface can get a much better performance. The hardware/software interface and operating system design in this paper are also very useful for the multi-core processor design.