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SHAKTI: An Open-Source Processor Ecosystem

Authors

 

Neel Gala G. S. Madhusudan

InCore Semiconductors Pvt. Ltd.

Paul George Anmol Sahoo Arjun Menon V. Kamakoti

Indian Institute of Technology, Madras

1.Introduction

 
Processors have become ubiquitous in all the appliances and machines we use, in both consumer and industrial settings. These processors range from extremely small and low power micro-controllers (used in motor controls, home robots and appliances) to high-performance multi-core processors (used in servers and supercomputers). However, the growth of modern AI/ML environments (like Caffe[Jia et al. 2014], Tensorflow[Abadi et al. 2016]) and the need for features like enhanced security has forced the industry to look beyond general purpose solutions and towards domain-specific-customizations. While a large number of companies today can develop custom ASICs (Application Specific Integrated Chips) and license specific silicon blocks from chip-vendors to develop a customized SoCs (System on Chips), at the heart of every design is the processor and the associated hardware. To serve modern workloads better, these processors also need to be customized, upgraded, re-designed and augmented suitably. This requires that vendors/consumers have access to appropriate processor variants and the flexibility to make modifications and ship them at an affordable cost.

Today, the processor market is dominated by just a few large companies like Intel, ARM and AMD. Each of these companies has an extensive portfolio of processors catering to various market segments. Of these only ARM has a model of licensing processors cores to other companies. Even when processor cores can be licensed, the license is usually very restrictive limiting changes to the ISA and typically has a strict NDA that prohibits publishing data like performance figures.

The struggle against such closed-source IP is not new. The software industry too has overcome the barriers of closed sourced products through the mass adoption of open source. The arrival of the world-wide-web in the 1990s made it possible to share code online, further boosting the growth of open-source software like Linux, GNU and Postgres. Today, the software community is armed with a veritable arsenal of open-source products which have been widely adopted. The hardware community however, has not seen such a revolution yet. An open-source processor eco-system will not only usher in the era of customized processors but also allow industry and academia to collaborate in creation of the next generation of processors. SHAKTI [Gala et al. 2016], an open-source initiative by IIT-Madras (Indian Institute of Technology, Madras) is aimed precisely at building such open-source processor development eco-systems, enabling community with enough resources to build customized, production grade processors, without the attendant hassle of licensing, NDAs, royalties or any other sort of restrictions.

 

1.1 The SHAKTI Processor Program

 
The SHAKTI Processor Program, was started as an academic initiative back in 2014 by the RISE Group at IIT-Madras. Seeking to overcome the limitations of the processor industry mentioned above, the initiative aims at not only creating open-source processors but also building associated components of the eco-system – interconnect fabrics, scalable verification platforms, peripheral IPs and software tools – thereby enabling rapid adoption. Some of the major highlights of the program are:

(1) Source code of all the HW components of the SHAKTI eco-system is open under the 3 part BSD license, the SW is released under GPL. This means a user could freely use, modify and circulate the source code without having to sign any NDAs, licenses or even notify the authors as long as the license terms are met. The SHAKTI program itself will not assert any patents and thereby removes the burden of patent encumbrance.

(2) The processors of the SHAKTI program are based on the open RISC-V [RISC-V 2015] ISA (In- struction Set Architecture). RISC-V has been designed for modularity and extensibility, enabling easy-customization and is supported by a comprehensive SW and tools eco-system. Since the ISA does not dictate micro-architectural features, the software and hardware can be maintained by two complete different entities and yet be compatible.

(3) The front-end (RTL – Register Transfer Level) design of the SHAKTI processors are developed in Bluespec, the open-source High Level Synthesis (HLS) language: Bluespec System Verilog (BSV) [Nikhil 2004]. BSV allows the user to develop using a paradigm of parameterized modules with well defined interfaces. There is also an open-source parser available, enabling anyone interested to build their own BSV compiler.

(4) The academic world now has access to a comprehensive family of real world implementations of processors. Typically academic research was restricted to the world of “simulators” and ”emulation models”. Now research results can be proven on actual processors using FPGAs or low-cost academic ASIC fabrication.

(5) An open-source eco-system such as SHAKTI promotes modular development environment where users can plug-in different open-source or proprietary IPs and innovate on new products and designs. Being completely open-source, it is exteremely difficult for external entities to add back-doors and trojans. This is of particular interest to the strategic sectors in countries like India, which today depend on black-box solutions provided by external entities.

In addition to the above advantages, a combination of an open-source processor eco-systems like SHAKTI and a semiconductor fab offering prototype facilities on its latest technology node allows small projects to produce custom processors.


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