Introduction

In the realm of hardware design and verification, the Peripheral Component Interconnect Express (PCIe) standard has become a cornerstone for high-speed data transfer between various hardware components. As the complexity of hardware systems increases, so does the necessity for rigorous testing methodologies to ensure the reliability and robustness of PCIe endpoints. One such advanced methodology involves modifying PCIe transactors in emulation environments to facilitate negative testing, which is crucial for identifying and addressing potential vulnerabilities and failure points.

Understanding PCIe Transactors

PCIe transactors serve as the interface between the testbench and the device under test (DUT) in emulation environments. They simulate the behavior of PCIe transactions, allowing for comprehensive testing of PCIe endpoints. Transactors can generate a wide range of valid and invalid PCIe transactions, thereby enabling the verification of compliance and robustness of the DUT.

The Role of Emulation in PCIe Verification

Emulation plays a pivotal role in the verification of modern hardware designs, offering a more efficient and scalable solution compared to traditional simulation methods. Emulation provides the ability to run extensive test scenarios at near-real-time speeds, making it possible to uncover issues that might be missed during simulation. This capability is particularly important for PCIe endpoints, where high-speed data transfer and complex protocols necessitate thorough testing.

Negative Testing: A Crucial Aspect of Verification

Negative testing involves deliberately introducing invalid or unexpected inputs to a system to observe its behavior under adverse conditions. This form of testing is essential for ensuring that the system can gracefully handle errors, recover from unexpected states, and maintain overall stability. For PCIe endpoints, negative testing can reveal vulnerabilities such as improper handling of malformed packets, incorrect error reporting, and failure to maintain data integrity.

Modifying PCIe Transactors for Negative Testing

To enable effective negative testing, PCIe transactors, and their associated tool APIs, must be modified to generate a variety of invalid and unexpected transactions. The TLP presents several opportunities to introduce negative testing.  We will examine a couple of these test scenarios:

1.    Setting the poison bit in TLP

How the PCIe endpoint handles a non-posted read with the TLP poison bit set is an important aspect to be tested.  There are both correctable and uncorrectable responses to errors such as this.  A PCIe endpoint maybe designed to pass on the poisoned data or it may need to report it as an uncorrectable, fatal or non-fatal, error.  Moreover, at a SoC level bus fabric connections in the path from say MMIO space or memory being read to the PCIe endpoint may handle poison in different ways.  It may pass on the poison data to the PCIe endpoint to handle and this would need to be tested.  Figure 2 shows the data flow and error response for such a test scenario:     

Creating specific test steps and test case definition is beyond the focus of this article but the following is a rough explanation of the test flow as shown in Figure 3:

1)      Emulation test code sets emulation environment tools API such that the PCIe transactor should set the poison bit for next non-posted read from the PCIe endpoint

2)      Emulation test code reads memory or MMIO space, TLP with data is read in I/O stack. 

3)      IP within the I/O stack may or may not allow the poisoned data to be proprogated to the PCIe endpoint under test.  This is another testing scenario at a SoC level that can be tested.

4)      PCIe endpoint acknowledges that the TLP poison bit is set.  PCIe endpoint AER registers are set and can be checked by emulation test code.

5)      PCIe endpoint generates a AER response and the response can be verified at the root port.

2.    Corrupting data payload such that it results in parity error at PCIe endpoint.

The following is a rough explanation of the test flow as shown in Figure 4 for parity error injection:

1)      Emulation test code sets emulation environment tools API such that the PCIe transactor should introduce a parity error for next non-posted read from the PCIe endpoint

2)      Emulation test code reads memory or MMIO space, TLP with data. 

3)      PCIe endpoint reads the data into its buffer and identifies parity error.  PCIe endpoint should see it as poisoned data.  PCIe endpoint AER registers are set and can be checked by emulation test code.

4)      PCIe endpoint generates a AER response and the response can be verified at the root port.

3. Enhancing Transactor Capabilities

We have identified two negative test scenarios in this article.  Once all the negative test scenarios have been identified, the capabilities of the PCIe transactors and associated tools must be enhanced to generate these scenarios. This can involve:

·       Modifying the packet generation logic to create malformed and invalid packets

·       Introducing mechanisms to corrupt data payloads and headers

·       Implementing timing violations by altering the timing characteristics of transactions

·       Adding support for generating unexpected sequence numbers and transaction types

4. Integrating with the Emulation Environment

The modified PCIe transactors must be seamlessly integrated with the emulation environment to enable comprehensive testing. This involves ensuring that the transactors can interact with the DUT and the testbench, providing accurate and meaningful results. Integration can also include the development of custom test scripts and scenarios that leverage the enhanced capabilities of the transactors.

5. Validating the Modifications

Before deploying the modified PCIe transactors for negative testing, it is essential to validate the modifications to ensure their correctness. This can involve running a series of baseline tests as part of model drop checkout to verify that the transactors generate the expected invalid transactions and that the DUT responds appropriately. Validation also includes ensuring that the modifications do not introduce any unintended side effects or errors.

Conclusion

This has just been an example of innovative verification.  Here At Approaching Zero Escapes Validation and Development; LLC, we strive to continue to increase verification and validation coverage.  And bring that new coverage to our clients.  You want zero bug escapes, to both internal customers and external customers.  At Approaching Zero Escapes, we want to partner with you to achieve that.

References

PCIe - TLP Header, Packet Formats, Address Translation, Config Space, Command Register, Configuration types