Effective Processor Verification with Logic Fuzzer Enhanced Co-simulation

The study on verification trends in the semiconductor industry shows that the design complexity is increasing, fewer companies achieve first silicon success and need more spins before production, companies hire more verification engineers, and 53% of the whole hardware-design-cycle is spent on the design verification [18]. The cost of a respin is high, and more than 40% of the cases that contribute to it are post-fabrication functional bug exposures  [16]. The study also shows that 65% of verification engineers’ time is spent on debug, test creation, and simulation  [17]. This paper presents a set of tools for RISC-V processor verification engineers that help to expose more bugs before production and increase the productivity of time spent on debugging, test creation and simulation. We present Logic Fuzzer (LF), a novel tool that expands the verification space exploration without the creation of additional verification tests. The LF randomizes the states or control signals of the design-under-test at the places that do not affect functionality. It brings the processor execution outside its normal flow to increase the number of microarchitectural states exercised by the tests. We also present Dromajo, the state of the art processor verification framework for RISC-V cores. Dromajo is an RV64GC emulator that was designed specifically for co-simulation purposes. It can boot Linux, handle external stimuli, such as interrupts and debug requests on the fly, and can be integrated into existing testbench infrastructure with minimal effort. We evaluate the effectiveness of the tools on three RISC-V cores: CVA6, BlackParrot, and BOOM. Dromajo by itself found a total of nine bugs. The enhancement of Dromajo with the Logic Fuzzer increases the exposed bug count to thirteen without creating additional verification tests.