Karthik Swaminathan

RISC-V processors have gained acceptance across a wide range of computing domains, from IoT to embedded/mobile class and even in server-class processing systems. In processing systems ranging from connected cars and autonomous vehicles, to those on-board satellites and spacecrafts, these processors are targeted to function in safety-critical systems, where Reliability, Availability and Serviceability (RAS)-based considerations are of paramount importance. Along with potential system vulnerabilities caused primarily due to random errors, these processors may also be sensitive to targeted errors, possibly from malicious entities, which raises serious concerns regarding the security and safety of the processing system. Consequently, such systems necessitate the incorporation of RAS-based considerations right from an early stage of processor design.

While the hardware and software ecosystem around RISC-V has been steadily maturing, there have, however, been limited developments in early stage reliability-aware design and verification. The Early-stage Reliability And Security Estimation for RISC-V (ERASER) tool attempts to address this shortcoming. It consists of an open source framework aimed at providing directions to incorporate such reliability and security features at an early, pre-silicon stage of design. These features may include what kind of protection to be applied and which components within the processor should they be applied to. The proposed infrastructure comprises of an open source toolchain for early stage modeling of latch vulnerability in a RISC-V core (SERMiner [1]), a tool for automated generation of stress marks that maximize the likelihood of a transient-failure induced error (Microprobe (RISC-V) [2]), and verification by means of statistical and/or targeted fault injection (Chiffre [3]). While the infrastructure is targeted towards any core that uses the RISC-V ISA, the repository provides an end-to-end flow for the Rocket core [4].

ERASER thus evaluates “RAS-readiness”, or the effectiveness of protection techniques in processor design such that processor vulnerability in terms of Failures-In-time (FIT) rate is minimized, for a specified power/performance overhead. FIT rate is defined as the number of failures in one billion hours of operation and is a standard vulnerability metric used in industry.

ERASER is an open source tool available for download at https://github.com/IBM/eraser. The tool currently supports analysis of all latches in the design across a single Rocket core and the generation of stressmarks that can be used to evaluate the vulnerability of these latches. In addition to radiation-induced soft errors, we plan to extend ERASER to also model errors due to voltage noise, thermal and aging-induced failures, both in memory and logic, and generate representative stressmarks.

ERASER is an initial effort to devise a comprehensive methodology for RAS analysis, particularly for open-source hardware, with the hope that it spurs further research and development into reliability-aware design both in industry and academia.

References:

1. K. Swaminathan, R. Bertran, H. Jacobson, P. Kudva, P. Bose, ‘Generation of Stressmarks for Early-stage Soft-error Modeling’, International Conference on Dependable Systems and Networks (DSN) 2019

2. S. Eldridge R. Bertran, A. Buyuktosunoglu, P. Bose, ‘MicroProbe: An Open Source Microbenchmark Generator, ported to the RISC-V ISA, the 7th RISC-V workshop, 2017

3. S. Eldridge, A. Buyuktosunoglu and P. Bose, ‘Chiffre A Configurable Hardware Fault Injection Framework for RISC-V Systems’ 2nd Workshop on Computer Architecture Research with RISC-V (CARRV), 2018

4. Krste Asanović, Rimas Avižienis, Jonathan Bachrach, Scott Beamer, David Biancolin, Christopher Celio, Henry Cook, Palmer Dabbelt, John Hauser, Adam Izraelevitz, Sagar Karandikar, Benjamin Keller, Donggyu Kim, John Koenig, Yunsup Lee, Eric Love, Martin Maas, Albert Magyar, Howard Mao, Miquel Moreto, Albert Ou, David Patterson, Brian Richards, Colin Schmidt, Stephen Twigg, Huy Vo, and Andrew Waterman, The Rocket Chip Generator, Technical Report UCB/EECS-2016-17, EECS Department, University of California, Berkeley, April 2016

The attached figure shows a representative flow for the RAS estimation methodology. An initial characterization of all instructions in the RISC-V ISA is carried out via RTL simulation using an existing core model (eg. the Rocket core). The simulation is configured to generate VCD (Value- Change Dump) files for every single instruction testcase. The SERMiner tool parses these VCD files to determine latch activities across the core, aggregated at a macro (or RTL module) level. Based on these per-instruction latch activities, SERMiner outputs an instruction sequence, which forms the basis of the SER stressmark to be generated by Microprobe (RISC-V). Microprobe (RISC-V) is a microbenchmark generation tool that is capable of generating microbenchmarks geared towards specific architecture and micro-architecture level characterization. One of its key applications is in the generation of stressmarks, or viruses, that target various worst-case corners of processor operation. These stressmarks may be targeted at maximizing power, voltage noise, temperature, or soft-error vulnerability as in case of this tool. The generated stressmark is then used to generate a list of latches that show a high residency and hence a high SER vulnerability. These latches are the focus of fault injection-based validation experiments using the Chiffre tool. Chiffre provides a framework for automatically instrumenting a hardware design with run-time configurable fault injectors. The vulnerable latches obtained from running the generated stressmarks through the Rocket core model, and then through SERMiner, are earmarked for targeted fault injection experiments using Chiffre. The objective of these experiments is to further prune the list of vulnerable latches by eliminating those that are derated, that is, they do not affect the overall output even when a fault is injected in them. Focusing any and all protection strategies on this final list of latches would maximize RAS coverage across the entire core.

Ongoing and future work:

ERASER currently only supports analysis of all latches in the design across a single Rocket core and the generated stressmarks can be used to evaluate the vulnerability of these latches. Most on-chip memory structures such as register files and caches, are equipped with parity/ECC protection and are as such protected against most radiation-induced soft errors. However, they are still vulnerable to supply voltage noise, thermal and aging-induced failures, and other hard or permanent errors. We plan to extend ERASER to model such errors, both in memory and logic, and generate stressmarks representative of worst-case thermal emergencies and voltage noise, in addition to soft errors.