Constraint-Based Automatic SBST Generation for RISC-V Processor Families

Abstract

Software-Based Self-Tests (SBST) allow at-speed, native online-testing of processors by running software programs on the processor core, requiring no Design for Testability (DfT) infrastructure. The creation of such SBST programs often requires time-consuming manual labour that is expensive and requires in-depth knowledge of the processor’s architecture to target hard-to-test faults. In contrast, encoding the SBST generation task as a Bounded Model Checking (BMC) problem allows using sophisticated, state-of-the-art BMC solvers to automatically generate an SBST. Constraints for the BMC problem are encoded in a circuit called Validity Checker Module (VCM) and applied during SBST generation.In this paper, we focus on presenting a VCM architecture and a constraint set that allows building SBSTs that make minimal assumptions about the firmware, targeting hard-to-test faults in the ALU and register file of multiple scalar, in-order RISC-V processor families. The VCM architecture consists of a processor-specific mapping layer and a generic constraint set connected via a well-defined interface. The generic constraint set enforces the desired SBST behaviour, including controlling the processor’s pipeline state, memory accesses, and with that executed instructions, register state, and fault propagations. Using a generic constraint set allows for rapid SBST generation targeting new RISC-V processor families while keeping the generic constraints untouched. Lastly, we evaluate this approach on two RISC-V processor families, namely the DarkRISCV and a proprietary, industrial core showing the portability and strength of the approach, allowing for rapidly targeting new processors.