ARM MMU Configuration Fails on Virt Board Due to Memory Layout Differences

ARM Cortex-A53 MMU Translation Fault on Virt Board During Bootloader Execution

The core issue revolves around a Memory Management Unit (MMU) configuration that works flawlessly on a Raspberry Pi 3B (Cortex-A53) but fails on a QEMU virt board (also Cortex-A53) with an "Instruction Abort" exception. The fault occurs immediately after enabling the MMU, specifically during the first instruction fetch post-MMU activation. The fault is identified as a Level 0 Translation Fault, indicating a failure in the initial stage of the MMU table walk. This discrepancy arises despite both systems using the same Cortex-A53 CPU and identical MMU configuration code. The root cause lies in the differences in memory system layouts between the two platforms, particularly the presence of ROM in the virt board’s memory map, which prevents the bootloader from writing to the page tables.

Memory Layout Mismatch and Translation Table Configuration

The primary cause of the MMU translation fault is a mismatch between the memory layout assumptions in the bootloader’s MMU configuration and the actual memory layout of the QEMU virt board. The Raspberry Pi 3B and the virt board, despite sharing the same Cortex-A53 CPU, have fundamentally different memory systems. The Raspberry Pi 3B has a contiguous memory region starting from address 0x0, allowing the bootloader to write page tables to this region without issues. In contrast, the virt board has a ROM region occupying the first gigabyte of memory, making this region read-only and inaccessible for writing page tables.

The translation tables, which describe the memory system to the processor, must align with the physical memory layout of the target platform. When the MMU is enabled, the processor performs a table walk to translate virtual addresses to physical addresses. If the translation tables are invalid or point to inaccessible memory regions, the MMU generates a translation fault. In this case, the fault occurs because the bootloader attempts to write page tables to a ROM region on the virt board, which is not writable. This results in a Level 0 Translation Fault, as the MMU cannot complete the table walk.

Additionally, the Exception Level (EL) and Security State differences between the two platforms contribute to the issue. The Raspberry Pi 3B boots into EL3 (Secure State), while the virt board boots into EL1 (Non-secure State). Although the bootloader drops to EL1 on both platforms, the initial EL3 configuration on the Raspberry Pi 3B may implicitly set up certain system registers, such as those related to EL2, which are not configured on the virt board. This discrepancy can lead to undefined behavior when the MMU is enabled.

Diagnosing and Resolving MMU Translation Faults on ARM Cortex-A53

To diagnose and resolve the MMU translation fault, follow these steps:

1. Verify Memory Layout and Translation Table Configuration
   Begin by examining the memory layout of the target platform. Use platform-specific documentation or debugging tools to identify the regions of memory that are writable and accessible during the bootloader phase. Ensure that the translation tables are written to a writable memory region. On the virt board, this may involve relocating the page tables to a region outside the ROM.

2. Check Exception Level and Security State Configuration
   Confirm that the Exception Level and Security State are consistent between the two platforms. If the Raspberry Pi 3B boots into EL3 and the virt board boots into EL1, ensure that any system registers configured in EL3 on the Raspberry Pi 3B are explicitly set up in EL1 on the virt board. This includes registers related to EL2, such as HCR_EL2 and VTCR_EL2, which may affect the MMU configuration.

3. Analyze ESR_EL1, FAR_EL1, and ELR_EL1 Registers
   When a translation fault occurs, the ESR_EL1 (Exception Syndrome Register) provides detailed information about the fault. Decode the ESR_EL1 value to determine the fault type and level. For example, a Level 0 Translation Fault indicates an issue with the initial stage of the table walk. The FAR_EL1 (Fault Address Register) and ELR_EL1 (Exception Link Register) provide the faulting address and the address of the instruction that caused the fault, respectively. Use these registers to pinpoint the exact location of the fault.

4. Validate Translation Table Entries and TCR Configuration
   Ensure that the translation table entries are correctly configured and aligned with the TCR (Translation Control Register) settings. The TCR defines the size and attributes of the address spaces managed by the translation tables. Verify that the T0SZ and T1SZ fields in the TCR match the address ranges defined in the translation tables. Additionally, check the EPDn fields to ensure that the translation table walks are not being prevented.

5. Implement Cache and TLB Maintenance Operations
   Before enabling the MMU, perform cache and TLB maintenance operations to ensure that the translation tables are coherent with the memory system. Use the TLBI VMALLE1IS instruction to invalidate the TLB and the IC IALLUIS instruction to invalidate the instruction cache. These operations prevent stale entries in the TLB and cache from causing translation faults.

6. Test and Validate on Both Platforms
   After making the necessary adjustments, test the MMU configuration on both the Raspberry Pi 3B and the virt board. Use debugging tools to verify that the translation tables are correctly written and that the MMU operates as expected. If the fault persists, revisit the memory layout and translation table configuration to identify any overlooked discrepancies.

By following these steps, you can diagnose and resolve MMU translation faults caused by memory layout mismatches and translation table configuration issues. The key is to thoroughly understand the memory system of the target platform and ensure that the MMU configuration aligns with the physical memory layout. This approach not only resolves the immediate issue but also provides a deeper understanding of the ARM Cortex-A53 MMU and its interaction with the memory system.

This comprehensive approach ensures that the MMU configuration is robust and platform-agnostic, enabling reliable operation across different ARM Cortex-A53-based systems.