Software-Based Flat Nested Page Table in Sunway Architecture

The nested page table (NPT) model is an effective, hardware-assisted memory virtualization solution. However, the current Sunway processor lacks hardware support of NPT. However, the privileged programmable interface of Sunway architecture can be used to emulate the necessary hardware support with software. Hardware mode is the CPU privilege level unique to Sunway. This interface runs on the Sunway hardware mode with the highest CPU privileged level. In this paper, we propose the software-based flat nested page table (swFNPT) model for Sunway. In the programmable interface, we software-implement the hardware functions required by the nested page table model, such as nested page table walking. The new design makes up for the deficiency in hardware support through software optimization. In particular, the flat (one-level) nested page table is used to improve the efficiency of page walk. We use multiple benchmarks to test the performance of swFNPT. The experiments on a Sunway 1621 server show the promising performance of swFNPT. The average memory virtualization overhead of SPEC CPU 2006 is about 3% and the average overhead for SPEC CPU 2017 benchmarks with large working set is about 4%. The STREAM result shows that the memory bandwidth loss of swFNPT is less than 3%. Therefore, this paper provides a valuable reference for future development of hardware-assisted virtualization of Sunway server.