ARM Cortex-A53 Misaligned Memory Access in Device Memory
The core issue revolves around a Data Abort exception occurring during a memcpy operation on an ARM Cortex-A53 processor. The exception is triggered specifically during a store pair (STP) instruction, which attempts to write data to a memory region marked as Device memory. The ARM Cortex-A53, being a 64-bit core, has specific alignment requirements for memory accesses, particularly when dealing with Device memory or when strict alignment checking is enabled. The STP instruction in question is attempting to store data to an address that is only 4-byte aligned, which violates the alignment requirements for Device memory accesses on a 64-bit core. This misalignment is detected by the hardware, leading to a Data Abort exception.
The Exception Syndrome Register (ESR_EL1) provides critical information about the nature of the exception. In this case, the ESR_EL1.ISS.DFSC field indicates an alignment fault. This field is part of the ARMv8-A architecture’s exception handling mechanism, which is designed to provide detailed information about the cause of an exception. The alignment fault is specifically triggered because the destination address for the STP instruction is not aligned to the required boundary for Device memory accesses. The ARM architecture mandates that accesses to Device memory must be aligned to the size of the element being accessed. For a 64-bit core like the Cortex-A53, this means that the address must be aligned to an 8-byte boundary when performing 64-bit accesses.
The shared memory sections in question are configured as uncacheable and marked as volatile, which further complicates the issue. Uncacheable memory regions, especially those marked as Device memory, have stricter alignment requirements compared to normal memory regions. This is because Device memory accesses are typically used for memory-mapped I/O, where misaligned accesses can lead to undefined behavior or hardware faults. The volatile keyword in C/C++ indicates that the memory location can be changed at any time by external factors, which means the compiler must avoid certain optimizations that might otherwise be applied. However, this does not affect the alignment requirements imposed by the hardware.
Misalignment in Device Memory and Strict Alignment Checking
The primary cause of the Data Abort exception is the misaligned memory access in a Device memory region. The ARM Cortex-A53 processor enforces strict alignment checking for Device memory accesses, meaning that any attempt to access Device memory with an address that is not aligned to the size of the element being accessed will result in an alignment fault. In this case, the STP instruction is attempting to store two 64-bit values to a memory address that is only 4-byte aligned. This violates the alignment requirements for Device memory accesses, leading to the Data Abort exception.
The alignment requirements for Device memory accesses are specified in the ARM Architecture Reference Manual. For a 64-bit core like the Cortex-A53, the address must be aligned to an 8-byte boundary when performing 64-bit accesses. This is because the hardware expects the address to be aligned to the natural boundary of the data being accessed. Misaligned accesses can cause the hardware to perform multiple memory transactions, which is not allowed for Device memory regions. Device memory is typically used for memory-mapped I/O, where each access must be atomic and aligned to the size of the element being accessed. Misaligned accesses can lead to undefined behavior, as the hardware may not be able to guarantee the atomicity of the access.
Another contributing factor is the configuration of the shared memory sections as uncacheable and volatile. Uncacheable memory regions are typically used for memory-mapped I/O, where the data is expected to change frequently and unpredictably. The volatile keyword in C/C++ ensures that the compiler does not optimize away memory accesses to these regions, but it does not affect the alignment requirements imposed by the hardware. The combination of uncacheable memory and strict alignment checking means that any misaligned access to these regions will result in an alignment fault.
The issue is further compounded by the use of the memcpy function, which does not guarantee alignment of the source and destination addresses. The memcpy function is designed to copy a block of memory from one location to another, but it does not perform any alignment checks or adjustments. If the source or destination address is misaligned, the memcpy function will still attempt to perform the copy, which can lead to alignment faults if the memory regions have strict alignment requirements. In this case, the destination address is misaligned, leading to the Data Abort exception during the STP instruction.
Implementing Proper Alignment and Memory Barrier Techniques
To resolve the Data Abort exception caused by misaligned memory access in Device memory, several steps must be taken to ensure proper alignment and correct memory barrier usage. The first step is to ensure that the destination address for the STP instruction is aligned to an 8-byte boundary. This can be achieved by adjusting the memory allocation or by using aligned memory access functions. For example, the posix_memalign function can be used to allocate memory that is guaranteed to be aligned to a specified boundary. Alternatively, the alignas keyword in C++ can be used to specify the alignment of a variable or structure.
Once the destination address is properly aligned, the next step is to ensure that the memcpy function is used correctly. The memcpy function should only be used for copying data between memory regions that do not have strict alignment requirements. For memory regions with strict alignment requirements, such as Device memory, it is recommended to use aligned memory access functions or to manually copy the data using aligned load and store instructions. For example, the ldp (load pair) and stp (store pair) instructions can be used to load and store 64-bit values from and to aligned memory addresses.
In addition to ensuring proper alignment, it is also important to use memory barriers to ensure that memory accesses are performed in the correct order. Memory barriers are used to enforce ordering constraints on memory operations, ensuring that certain operations are completed before others begin. This is particularly important when dealing with Device memory, where the order of memory accesses can affect the behavior of the hardware. The ARM architecture provides several memory barrier instructions, such as dmb (data memory barrier), dsb (data synchronization barrier), and isb (instruction synchronization barrier). These instructions can be used to enforce ordering constraints on memory accesses, ensuring that the STP instruction is executed only after all previous memory accesses have completed.
Finally, it is important to review the memory attributes and access permissions for the shared memory sections. The memory attributes, such as cacheability and shareability, can affect the behavior of memory accesses and should be configured correctly for the intended use case. For example, memory regions used for memory-mapped I/O should be marked as Device memory and uncacheable, while memory regions used for data storage should be marked as Normal memory and cacheable. The access permissions, such as read/write permissions, should also be configured correctly to prevent unauthorized access to memory regions.
In conclusion, the Data Abort exception on the ARM Cortex-A53 is caused by a misaligned memory access in a Device memory region. To resolve this issue, it is necessary to ensure proper alignment of the destination address, use aligned memory access functions, and implement memory barriers to enforce ordering constraints on memory accesses. By following these steps, the Data Abort exception can be avoided, and the system can operate correctly.
