ARM Cortex-A72 Exclusive Access and Cache Coherency Challenges
The issue at hand revolves around an SError interrupt triggered by the LDAXRB instruction when the data cache is disabled on the NXP LS1046A platform, which utilizes the ARM Cortex-A72 processor. The LDAXRB instruction is a load-exclusive operation, part of ARM’s exclusive access mechanism used for synchronization primitives like atomic operations. The problem manifests specifically when the data cache is disabled, as indicated by the modification of the SCTLR_EL3 register from 0x00c5183d to 0x00c51839. This modification clears the C bit, which controls the data cache, effectively disabling it.
The LDAXRB instruction performs a load operation while setting up the Exclusive Monitor, a hardware mechanism that tracks exclusive accesses. When the cache is enabled, the Exclusive Monitor functionality is typically handled within the L1 cache using cache coherence protocols. However, when the cache is disabled, the core relies on the memory system to handle exclusive access monitoring. This reliance on the memory system introduces a dependency on the interconnect and memory controller’s ability to support exclusive access transactions.
The SError interrupt occurs because the memory system or interconnect does not support exclusive access transactions for the specific memory region being accessed. This is evidenced by the fact that replacing the LDAXRB instruction with a regular load instruction (LDR) eliminates the SError interrupt. The Cortex-A72 Technical Reference Manual (TRM) confirms that the Exclusive Monitor behavior differs based on the memory type and cache state. For cacheable memory regions, the Exclusive Monitor is handled internally by the L1 cache. For non-cacheable or device memory regions, the Exclusive Monitor must be supported externally by the interconnect or memory controller.
The NXP LS1046A platform uses the CCI-400 interconnect, which claims to support the External Global Monitor required for exclusive access transactions. However, the absence of explicit documentation in the LS1046A reference manual regarding the support for exclusive access transactions in non-cacheable memory regions suggests a potential gap in the implementation or documentation. Additionally, the CCI-400 documentation mentions support for exclusive access transactions but also notes that certain lock transactions are terminated at the CCI level, as the system beyond the CCI does not support locked transactions. This discrepancy indicates that the memory system’s support for exclusive access transactions may be limited or inconsistent.
Memory System Limitations and Exclusive Access Support
The root cause of the SError interrupt lies in the memory system’s inability to support exclusive access transactions when the cache is disabled. The ARM Cortex-A72 processor relies on the Exclusive Monitor to implement atomic operations, and the behavior of the Exclusive Monitor varies depending on the memory type and cache state. For cacheable memory regions, the Exclusive Monitor is handled internally by the L1 cache, leveraging cache coherence protocols to ensure atomicity. However, for non-cacheable memory regions, the Exclusive Monitor must be supported externally by the interconnect or memory controller.
The NXP LS1046A platform uses the CCI-400 interconnect, which is designed to support exclusive access transactions. However, the CCI-400’s support for exclusive access transactions is contingent on the memory system’s ability to handle these transactions. The CCI-400 documentation indicates that it supports exclusive access transactions but also notes that certain lock transactions are terminated at the CCI level, as the system beyond the CCI does not support locked transactions. This suggests that the memory system’s support for exclusive access transactions may be limited or inconsistent.
The absence of explicit documentation in the LS1046A reference manual regarding the support for exclusive access transactions in non-cacheable memory regions further complicates the issue. While the CCI-400 interconnect claims to support the External Global Monitor required for exclusive access transactions, the lack of detailed documentation makes it difficult to determine whether the memory system fully supports these transactions. This ambiguity is a significant factor in the SError interrupt triggered by the LDAXRB instruction when the cache is disabled.
The Cortex-A72 TRM provides additional context for understanding the issue. It states that for cacheable memory regions, the Exclusive Monitor is handled internally by the L1 cache, using cache coherence protocols to ensure atomicity. However, for non-cacheable memory regions, the Exclusive Monitor must be supported externally by the interconnect or memory controller. This external support is critical for ensuring that exclusive access transactions are handled correctly when the cache is disabled. The SError interrupt occurs because the memory system or interconnect does not fully support exclusive access transactions for the specific memory region being accessed.
Implementing Cache-Aware Synchronization and Memory Barrier Strategies
To address the SError interrupt triggered by the LDAXRB instruction when the cache is disabled, several strategies can be employed to ensure proper synchronization and memory access behavior. These strategies involve leveraging cache-aware synchronization techniques, implementing memory barriers, and ensuring that the memory system supports exclusive access transactions.
First, it is essential to ensure that the memory system supports exclusive access transactions for the specific memory region being accessed. This can be achieved by verifying the memory system’s capabilities and ensuring that the interconnect and memory controller fully support exclusive access transactions. If the memory system does not support exclusive access transactions, alternative synchronization mechanisms may need to be employed, such as using spinlocks or other software-based synchronization primitives.
Second, implementing cache-aware synchronization techniques can help mitigate the issue. When the cache is enabled, the Exclusive Monitor is handled internally by the L1 cache, leveraging cache coherence protocols to ensure atomicity. By ensuring that the memory region being accessed is cacheable, the Exclusive Monitor can function correctly, and the LDAXRB instruction can be used without triggering an SError interrupt. This approach requires careful management of the cache state and ensuring that the memory region is marked as cacheable in the page tables.
Third, implementing memory barriers can help ensure proper memory access ordering and synchronization. Memory barriers are instructions that enforce ordering constraints on memory operations, ensuring that certain operations are completed before others. In the context of exclusive access transactions, memory barriers can be used to ensure that the Exclusive Monitor is properly set up before the LDAXRB instruction is executed. This can help prevent race conditions and ensure that the exclusive access transaction is handled correctly.
Finally, if the memory system does not support exclusive access transactions, alternative synchronization mechanisms may need to be employed. One approach is to use spinlocks, which are software-based synchronization primitives that rely on busy-waiting to ensure mutual exclusion. While spinlocks can be less efficient than hardware-based synchronization mechanisms, they can provide a reliable alternative when exclusive access transactions are not supported.
In summary, addressing the SError interrupt triggered by the LDAXRB instruction when the cache is disabled requires a combination of strategies, including ensuring that the memory system supports exclusive access transactions, implementing cache-aware synchronization techniques, using memory barriers, and employing alternative synchronization mechanisms if necessary. By carefully managing the cache state and ensuring proper synchronization, the issue can be mitigated, and the LDAXRB instruction can be used without triggering an SError interrupt.
