Open-Source HW/SW Co-Simulation Using QEMU and GHDL for VHDL-Based SoC Design

Hardware/software co-simulation is a technique that can help design and validate digital circuits controlled by embedded processors. Co-simulation has largely been applied to system-level models, and tools for SystemC or SystemVerilog are readily available, but they are either not compatible or very cumbersome to use with VHDL, the most commonly used language for FPGA design. This paper presents a direct, simple-to-use solution to co-simulate a VHDL design together with the firmware (FW) that controls it. It aims to bring the power of co-simulation to every digital designer, so it uses open-source tools, and the developed code is also open. A small patch applied to the QEMU emulator allows it to communicate with a custom-written VHDL module that exposes a CPU bus to the digital design, controlled by the FW emulated in QEMU. No changes to FW code or VHDL device code are required: with our approach, it is possible to co-simulate the very same code base that would then be implemented into an FPGA, enabling debugging, verification, and tracing capabilities that would not be possible even with the real hardware.