Accelerate Energy Efficient Compute Designs with Synopsys CXL 3.0 Verification IP

As high-performance computing (HPC) and artificial intelligence (AI) workloads scale in complexity and volume, energy efficiency has become a defining challenge for data centres worldwide. Enter Compute Express Link (CXL) 3.0—a groundbreaking interconnect standard that is reshaping how memory and compute resources are pooled, shared, and managed across heterogeneous systems. Among its most impactful innovations is the L0p low-power operational state, designed to maximize energy savings without sacrificing bandwidth or responsiveness. In this post, we’ll explore the evolving role of L0p in CXL 3.0, its importance in modern computing architectures, and how Synopsys Verification IP (VIP) empowers engineers to validate these features for next-generation deployments.

Compute Express Link (CXL) 3.0 is the backbone of next-gen HPC and AI systems, enabling seamless, high-speed, low-latency communication between CPUs, memory, accelerators, and other devices. The three protocols—CXL.io, CXL.cache, and CXL.mem—work together to enable resource sharing, memory pooling, and cache coherence, which are vital for cloud-scale and AI-centric environments.

L0p is a strategic low-power state where devices remain operational with reduced energy draw. Unlike deep sleep states that incur latency penalties, L0p enables rapid transitions back to full performance (L0) as soon as workload demands spike—a crucial capability for bursty AI inference or fluctuating HPC simulations. The Figure 1 illustrates a high-level architecture for HPC and AI acceleration using CXL 3.0 L0p. It shows CPUs connected to switches, which in turn link to CXL 3.0 devices. 

Here are some key characteristics that make the L0p low-power state valuable in CXL 3.0 devices:

• Active but Energy-Efficient: In L0p, the CXL 3.0 device is still active and can perform operations but at a reduced power level compared to the full-power L0 state.
• Transition from L0: A device typically transitions from the fully operational L0 state to L0p when idle or less demanding workloads are present. This enables power savings without fully putting the device into a low-power, non-operational state like L1.
• Quick Recovery: One of the major advantages of L0p is its quick recovery time. Devices in L0p can quickly transition back to L0 (full performance mode) when higher processing power is needed, ensuring minimal performance penalties.

With memory pooling, CXL 3.0 enables flexible allocation of memory across CPUs and accelerators. L0p ensures that devices are always ready to serve memory requests without unnecessary power drain. For example, in cloud-native environments, L0p allows accelerators to “wake up” instantly when shared memory is needed for real-time inference or training.

The combination of L0p and CXL 3.0 is particularly beneficial for applications that require high performance but are sensitive to energy consumption:

• AI/ML: L0p facilitates energy savings in data centers running transformer models or generative AI workloads.
• HPC: Scientific simulations, such as climate modeling, leverage L0p to keep thousands of interconnected devices responsive yet efficient.

• Cloud Data Centers: CXL 3.0 with L0p supports elastic scaling while maintaining green computing standards.

Verification challenges continue to grow with each generation of evolving protocols. Specific challenges for CXL 3.0 L0p include:

• Complex Power State Transitions: Ensuring correct entry and exit timing for L0p under dynamic HPC/AI workloads.
• Data Integrity and Protocol Compliance: Verifying that data remains consistent and protocol rules are followed during low-power operation.
• Reset, Error Recovery, and Traffic Bursts: Handling unexpected resets, error recovery sequences, and high traffic bursts while in L0p state.
• Corner Cases, Simultaneous & Stress Conditions: Testing rare scenarios and extreme conditions to ensure robust behaviour under all circumstances.
• Validating sequential low-power state transitions (L0p → L1) without impacting performance or compliance.

Synopsys CXL 3.0 VIP offers a comprehensive suite for L0p verification, supporting the latest CXL 3.0 and PCIe specs. It leverages ARB/MUX ALMPs for dynamic link width negotiation, allowing for priority-based requests and real-time width adjustments based on aggregate demands. Engineers can orchestrate L0p transitions, configure NAK responses for robustness testing, and monitor trace logs for detailed debugging.

Synopsys CXL 3.0 VIP offers several features to streamline L0p verification:

• Configurable L0p Request: The Synopsys CXL 3.0 VIP provides an easy-to-use API interface that allows verification teams to initiate L0p requests on demand during simulation. This capability is designed to support test scenarios where dynamic link width changes are required to validate power management behaviour.
• Configurable NAK for L0p Request: CXL 3.0 VIP provides flexibility in handling L0p requests by allowing users to configure Negative Acknowledgment (NAK) responses. This feature enables verification teams to test corner cases and robustness of link management logic, ensuring that the system correctly handles retries, fallback states, and timing constraints when NAK responses occur.
• Simultaneously Request Handling: CXL 3.0 VIP provides flexibility and resolve If both ports are simultaneously requesting link width changes.
• Trace Logging and Debug: CXL 3.0 VIP provides a detailed log of the L0p request and response flow during simulation. It helps visualize and analyse how the system negotiates link width changes and transitions between states. For example, when the host sends an ALMP_L0P request and the device responds, these steps are captured in the trace for debugging.

• In-built Checks: These are automated validation mechanisms within the VIP that confirm whether the L0p request and response sequences follow the CXL 3.0 specification.

Synopsys CXL 3.0 Verification IP offers industry-leading solutions for verifying new architectural features, Handling Higher Bandwidth & Speed, and ensuring compliance with latest CXL 3.0 standards. Key advantages include: 

• Comprehensive protocol checks and functional coverage
• Reduces simulation time significantly and enables early tapeout through optimized testbench architecture and shift-left methodologies.
• Native Debug Integration: Verdi® protocol and memory-aware debug, plus Verdi® Performance Analyzer for performance metrics.

CXL 3.0’s L0p state is a game-changer for energy-efficient, high-performance computing environments. By intelligently balancing power and performance, L0p enables sustainable growth for AI and HPC workloads. Synopsys CXL 3.0 VIP provides a comprehensive solution for engineers to debug and validate the L0p feature efficiently. It offers flexibility to create dynamic and complex corner-case scenarios, enabling thorough verification of power management behaviour under various conditions. By supporting configurable L0p requests, NAK handling, and simultaneous request resolution, the VIP ensures robust testing of link width negotiation and fallback mechanisms. Combined with trace logging and in-built compliance checks, it accelerates the verification process while maintaining adherence to the CXL 3.0 specification. 

Synopsys is partnering with early customers and collaborators to enhance the standard architecture for their next-generation designs, incorporating new features now available with the latest specifications.  

Synopsys VIP is natively integrated with the Synopsys Verdi® Protocol Analyzer debug solution as well as Synopsys Verdi® Performance Analyzer. Running system-level payload on SoCs requires a faster hardware-based pre-silicon solution. Synopsys transactors, memory models, hybrid and virtual solutions based on Synopsys IP enable various verification and validation use-cases on the industry’s fastest verification hardware, Synopsys ZeBu® emulation and Synopsys HAPS® prototyping systems.