Count overflow and privilege mode filtering extension implementation on a RISC-V on-board processor

RISC-V is a computer architecture that has recently attracted considerable attention due to its advantageous qualities: it is an open instruction set, based on reduced and simple instructions. For this reason it has become an appealing choice for a wide range of computing applications and has positioned it as a disruptive force in a wide variety of fields, including those that involve the development of safety–critical software, as in the space sector. The ability to evaluate the activities performed within a processor is of paramount importance in this type of systems to ensure the fulfillment of the requirements during space missions. The monitoring of these events inside the processor is managed by an instrument called Hardware Performance Monitor (HPM). This work shows the implementation of the Sscofpmf extension of the HPM compliant to the RISC-V privileged specification. The paper details the redesign of the existing performance counters from a RISC-V baseline version previously implemented. A comparison between the two versions of both resource utilization data and power consumption is also provided. As expected, the Sscofpmf extension version has a higher resource utilization. Nevertheless, the paper shows that the additional functionalities included in the system have been validated without any changes in the processor clock frequency, so the extension does not introduce any performance overhead.