Скачать или смотреть Solutions for Data Cache Invalidation in User Space on Linux for Cortex A53

Discover how to effectively manage `data cache invalidation` and flush on Linux with ARM Cortex A53 while navigating the constraints of heterogeneous non-coherent caches.
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Understanding Data Cache Invalidation on Linux for ARM Cortex A53

If you're working on embedded systems or applications utilizing the ARM Cortex A53 architecture, you might find yourself faced with the challenge of managing data caches, especially when implementing a solution for data cache invalidation or flushing. This concern is amplified in environments where you have heterogeneous, non-coherent caches, such as when your Cortex A53 system interfaces with ARM M7 cores. In this guide, we'll delve into the problem and provide you with insights on how to tackle this issue effectively.

The Challenge

In a typical scenario, when running an SMP (Symmetric Multi-Processing) Linux operating system, many users seek ways to:

Invalidate or flush the data caches (L1/L2) from user space.

Ensure that operations on data do not inadvertently remove valuable cache lines, thus requiring precise cache manipulation.

For the Cortex A53, while it's clear that flushing or invalidating caches is a necessity, there are significant limitations to consider, particularly in user-mode. Users often inquire whether there are hidden APIs available for this purpose, especially when dealing with kernel versions like Linux 5.10.120.

The Solutions Available

1. Access Restrictions in User Space

First and foremost, it’s essential to understand that if you are running 32-bit user-space software, you're out of luck. You cannot access data cache maintenance operations from user space directly. This restriction poses a considerable hurdle for developers looking to fine-tune their data management strategies.

2. Aarch64 64-Bit Mode Benefits

However, if you are running in Aarch64 64-bit mode and the kernel has enabled user space access (SCTLR_EL1.UCI and SCTLR_EL2.UCI set to 1), you can utilize specific cache maintenance instructions. Among these are:

DC CVAU (Data Cache: Clean by VA to Uncached Write)

DC CIVAC (Data Cache: Clean and Invalidate by VA to PoC)

DC CVAC (Data Cache: Clean by VA to PoC)

These instructions allow user-space applications to manage cache more effectively, albeit with some degree of limitation.

3. Fine-Grained Control with Builtins

Using __builtin___clear_cache, developers have noted its applicability mostly for instruction caches. Despite its utility, when it comes to needing better granularity for data caches, further granularity is limited:

While the __builtin___clear_cache allows specifying a start and end, you still may need to iterate through cache lines for cache management. In the context of 64-bit ARM, this means iterating over every 64-byte chunk of your buffer to clean/invalidate line-by-line when using the accessible by-VA instructions.

4. Alternative Techniques for Performance

Depending on the size of the data being processed, it might be more efficient to set up a non-cached DMA (Direct Memory Access) region. This approach can significantly reduce the overhead associated with cache maintenance operations, as it bypasses the cache altogether for specific memory accesses.

Conclusion

Managing data cache invalidation and flushing in user mode on a Linux system utilizing ARM architecture, such as the Cortex A53, requires a clear understanding of the restrictions in play. Moving forward, ensure that you're aware of the required operating conditions and consider leveraging DMA for large data transfers to improve performance. By approaching cache management with a strategy that combines the allowed instructions in Aarch64 and an understanding of your system's architecture, you can effectively navigate the constraints and achieve your desired outcomes.