Building a Swiss cheese model approach for processor verification

3 slices of cheese handing

Building a Swiss cheese model approach for processor verification

Processors all have high quality requirements and their reliability is the main concern of processor verification teams. Providing best-in-class quality products requires a strategic, diligent and thorough approach. Processor verification therefore plays a major role and it takes a combination of all industry standard techniques – like in a Swiss cheese model.

The need for a strong, layered processor verification strategy

You’ve heard me say this before: processor verification is a subtle art. We need to take into account uncertainty, which means opening the scope of our verification while optimizing resources. On one hand, we want to find all critical bugs before final production, and on the other hand we must have an efficient verification strategy to fulfill time to market requirements. Producing smart processor verification means finding meaningful bugs as efficiently and as early as possible during the development of the product. One way of achieving this consists in combining all industry standard verification techniques. It is by creating redundancy that we find all critical bugs. 

There are different types of bugs and each bug has a complexity – or bug score – that depends on the number of events and types of events required to trigger the bugs. Some might be found with coverage, others with formal proofs, etc. Imagine the Swiss cheese model applied to processor verification. Each slice of cheese is a verification technique which has some specific strengths to catch some categories of bugs. The risk of a bug escaping and making it into the end product is mitigated by the different layers and types of verification which are layered behind each other. 

In a Swiss cheese model applied to processor verification, the principle is similar to the aviation industry: if there is a direct path going through all the slices, then there is a risk of plane crash. That is why the aviation industry is strict about procedures, checklists, and redundant systems. The objective is to add more slices and reduce the size of the holes on a slice so that in the end, there is no hole going through, and we deliver a quality processor.

Layers of swiss cheese models showing verification techniques

Swiss cheese model principles applied to processor verification

By using several slices of cheese, or verification methods:

    • We create redundancy to ensure continuity if one of the layers fails.
    • When bugs are found during development, it indicates that there was a hole in several slices. So we can improve several verification methods and reduce the size of the holes in each slice. In doing that, we increase our chances to hit bugs, from silly bugs to corner cases and from simple to complex bugs.
    • We maximize the potential of each technique.

A hole in a slice is a hole in the verification methodology. The more holes, and the bigger the holes, the more bugs can escape. If the same area of the design (overlapping holes between cheese slices) is not covered and tested by any of the verification techniques, then the bug will make it through and end up in the final deliverables.

A good verification methodology must present as few holes as possible, as small as possible, on each slice. A solid strategy, experience, and efficient communication are important factors to deliver quality products. 

When we find a bug, or a hole in a slice, during verification, we always fix it and check other slices for similar holes. Every slice should find the holes in the previous one and address them before progressing. Sanity checks are an efficient way to achieve this, for example by comparing our design with industry standard models such as Spike or Imperas.

Swiss cheese model slices showing processor verification quality

In the Swiss cheese model applied to processor verification, if one technique is strengthened – an improved testbench, new assertions, etc. – the bug is found and fixed before the product goes into production. All processor verification techniques are important and it is the combination of all of them that makes each of them more efficient. 

A single verification technique cannot do everything by itself, it is the action of all of them that improves the overall quality of the verification and processor design. There can be unexpected changes or factors during the development of a product, external actions that can impact the efficiency of a technique. For example, a change in the design not communicated to the verification team or a difficult Friday afternoon leading to human mistakes. These factors can increase the size of a hole in a slice, hence the importance of having more than one – and the importance of keeping engineering specifications up to date and communicating regularly between designers and verification engineers.  Code reviews conducted by other team members is one efficient solution to achieve this, and that is what we do at Codasip.

At Codasip, we use verification technology and techniques that allows us to create redundancy, preventing holes to go through the pile of slices of cheese, and to deliver best-in-class RISC-V processors.

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Philippe Luc

Verification Director

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