ARM Cortex-A53 Cache Flush by Physical Address: Challenges and Solutions

ARM Cortex-A53 Cache Architecture and Physical Address Flush Limitations

The ARM Cortex-A53 processor, a widely used 64-bit ARMv8-A core, employs a sophisticated cache architecture designed to optimize performance while maintaining coherency across multiple levels of caching. The Cortex-A53 features separate Level 1 (L1) instruction and data caches, as well as a unified Level 2 (L2) cache. The L1 data cache is physically indexed and physically tagged (PIPT), while the L1 instruction cache is virtually indexed and physically tagged (VIPT). The L2 cache is physically indexed and tagged. This architecture introduces complexities when attempting to perform cache maintenance operations, such as flushing, based on physical addresses (PA).

The primary challenge arises from the fact that ARM’s cache maintenance operations are typically designed to work with virtual addresses (VA). This is because the processor’s memory management unit (MMU) translates virtual addresses to physical addresses, and the cache maintenance instructions are optimized for this workflow. However, in certain scenarios, such as when working with Direct Memory Access (DMA) controllers or other processing elements that operate solely on physical addresses, there is a need to flush caches based on physical addresses. Unfortunately, ARM does not provide a direct mechanism to flush caches using physical addresses, as the cache maintenance operations are inherently tied to virtual addresses.

The Cortex-A53’s cache architecture further complicates this issue. Since the L1 data cache is physically indexed, it is theoretically possible to determine the cache set associated with a given physical address. However, the cache way cannot be determined without additional information, as multiple ways within a set may contain data corresponding to the same physical address. This means that flushing a specific cache line based solely on a physical address requires flushing all ways within the relevant set, which can lead to performance inefficiencies.

DMA Coherency and Cache Maintenance Challenges

One of the most common scenarios where physical address-based cache flushing is required is when working with DMA controllers. DMA controllers operate directly on physical addresses, bypassing the CPU’s MMU and caches. This can lead to coherency issues if the data being transferred by the DMA controller is also present in the CPU’s caches. For example, if a DMA controller is programmed to read data from a memory region that is cached in the CPU’s L1 or L2 cache, the DMA controller may read stale data from memory unless the CPU’s caches are flushed beforehand.

In such cases, the system software must ensure that the relevant cache lines are flushed to memory before initiating the DMA transfer. However, since the DMA controller operates on physical addresses, the software must map these physical addresses to virtual addresses to perform the necessary cache maintenance operations. This mapping is not always straightforward, especially in systems with complex memory management schemes, such as those involving multiple virtual address spaces or non-contiguous physical memory regions.

Furthermore, the Cortex-A53’s cache coherency mechanisms add another layer of complexity. The processor supports cache coherency across multiple cores within a cluster, but this coherency is managed at the hardware level and is not directly accessible to software. When performing cache maintenance operations, software must consider whether the operation needs to be broadcast to all cores in the cluster or if it can be performed locally. This decision depends on the specific cache maintenance instruction being used and the system’s coherency configuration.

Implementing Physical Address-Based Cache Flushing on Cortex-A53

To address the challenges of physical address-based cache flushing on the Cortex-A53, system software must implement a combination of address translation and cache maintenance operations. The following steps outline a practical approach to achieving this:

1. Address Translation: The first step is to translate the physical address to a virtual address. This can be done using the ARMv8-A address translation instructions, such as the AT instruction, which performs a virtual-to-physical address translation and stores the result in the PAR_EL1 register. The software can then use the translated virtual address to perform cache maintenance operations.

2. Cache Maintenance Operations: Once the virtual address is obtained, the software can use ARMv8-A cache maintenance instructions to flush the relevant cache lines. The specific instruction to use depends on the desired operation (e.g., clean, invalidate, or clean and invalidate) and the cache level (L1 or L2). For example, the DC CVAC instruction cleans a cache line by virtual address to the point of coherency, while the DC CIVAC instruction cleans and invalidates a cache line by virtual address to the point of coherency.

3. Broadcasting Cache Maintenance Operations: In systems with multiple cores, it may be necessary to broadcast cache maintenance operations to ensure coherency across all cores. This can be done using the appropriate cache maintenance instructions that support broadcasting, such as DC CISW (clean and invalidate by set/way) or DC CVAC (clean by virtual address to the point of coherency). The software must ensure that these operations are performed on all cores that may have cached the relevant data.

4. Handling Non-Coherent DMA: In systems where the DMA controller is non-coherent, additional steps may be required to ensure data consistency. This may involve flushing the entire cache or specific cache sets/ways before initiating the DMA transfer. The software must carefully manage these operations to avoid unnecessary performance overhead.

5. Optimizing Performance: To minimize the performance impact of cache flushing, the software should aim to perform these operations only when necessary. This can be achieved by tracking which memory regions are being accessed by DMA controllers and flushing only the relevant cache lines. Additionally, the software can leverage the Cortex-A53’s cache coherency mechanisms to reduce the need for explicit cache maintenance operations.

The following table summarizes the key cache maintenance instructions and their applicability to physical address-based flushing:

In conclusion, while the ARM Cortex-A53 does not provide a direct mechanism for flushing caches based on physical addresses, system software can achieve this by combining address translation with appropriate cache maintenance operations. This approach requires careful consideration of the processor’s cache architecture, coherency mechanisms, and the specific requirements of the system’s DMA controllers. By following the outlined steps and leveraging the available cache maintenance instructions, developers can ensure data consistency and optimize system performance in scenarios requiring physical address-based cache flushing.