Synchronous Exceptions at EL0 During User Program Execution
When transitioning from EL1 (kernel mode) to EL0 (user mode) on an ARM Cortex-A processor, synchronous exceptions can occur unexpectedly during the execution of user-space instructions. These exceptions are often accompanied by an Exception Syndrome Register (ESR) value of 0x2000000, indicating an "Unknown reason" with an instruction length (IL) of 1. The Fault Address Register (FAR_EL1) may also contain seemingly invalid values, such as 0xffff_ffff_ffff_fffc, further complicating the diagnosis. The issue is particularly perplexing because the memory mappings and accessibility of the addresses involved appear correct, as evidenced by successful store and load operations preceding the exception.
The problem manifests consistently at specific instructions, such as STR or LDR, and the exact address of the faulting instruction can vary depending on the state of the instruction and data caches. Disabling the instruction cache can shift the point of failure to immediately after the ERET instruction, suggesting that cache coherency plays a critical role in this behavior. The stack pointer (SP) and program counter (PC) values at the time of the exception are typically valid, and the memory mappings for these addresses are correctly configured in the Translation Table Base Register 0 (TTBR0).
Cache Coherency and Memory Barrier Omissions
The root cause of these synchronous exceptions lies in the improper handling of cache coherency and memory barriers during the transition from EL1 to EL0. When the kernel modifies the memory mappings in TTBR0 and invalidates the Translation Lookaside Buffer (TLB), it must also ensure that the data and instruction caches are properly synchronized. Failure to do so can result in the processor executing stale or incorrect instructions, leading to unexpected exceptions.
The ARM architecture relies on a weakly ordered memory model, meaning that memory operations can be reordered by the processor unless explicit memory barriers are used. When switching from kernel mode to user mode, the following sequence of events must be carefully managed:
- Modification of TTBR0: The kernel updates the translation tables to map the user program into the lower memory region.
- TLB Invalidation: The kernel invalidates the TLB entries to ensure that the new mappings are used.
- Data Cache Invalidation: The kernel must invalidate the data cache to ensure that any cached data corresponding to the new mappings is cleared.
- Instruction Cache Invalidation: The kernel must invalidate the instruction cache to ensure that the processor fetches the correct instructions from memory.
- Memory Barriers: The kernel must use appropriate memory barriers to ensure that the cache invalidations and TLB invalidations are completed before the ERET instruction is executed.
If any of these steps are omitted or performed out of order, the processor may attempt to execute stale instructions or access invalid data, resulting in synchronous exceptions. The ESR value of 0x2000000 indicates that the processor encountered an invalid instruction or data access, but the exact cause is not immediately apparent due to the complexity of the memory hierarchy.
Implementing Cache Invalidation and Memory Barriers
To resolve the issue of synchronous exceptions at EL0, the kernel must implement a robust cache invalidation and memory barrier strategy. The following steps outline the necessary actions to ensure proper cache coherency and memory synchronization:
1. Data Cache Invalidation
Before transitioning to EL0, the kernel must invalidate the data cache to ensure that any cached data corresponding to the new memory mappings is cleared. This can be achieved using the DC IVAC (Data Cache Invalidate by Virtual Address to Point of Coherency) instruction. The kernel should iterate over the relevant memory regions and issue DC IVAC for each cache line.
// Example of data cache invalidation
mrs x0, DCZID_EL0 // Read the data cache line size
and x0, x0, #0xF // Extract the log2 of the cache line size
mov x1, #1
lsl x1, x1, x0 // Calculate the cache line size in bytes
// Invalidate the data cache for the user program's memory region
mov x2, #0x0 // Start address of the user program
mov x3, #0x1000 // Size of the user program's memory region
1:
dc ivac, x2 // Invalidate the cache line at address x2
add x2, x2, x1 // Move to the next cache line
cmp x2, x3
b.lt 1b // Repeat until all cache lines are invalidated
2. Instruction Cache Invalidation
After invalidating the data cache, the kernel must also invalidate the instruction cache to ensure that the processor fetches the correct instructions from memory. This can be done using the IC IALLU (Instruction Cache Invalidate All to Point of Unification) instruction, which invalidates the entire instruction cache.
// Example of instruction cache invalidation
ic iallu // Invalidate the entire instruction cache
isb // Ensure the invalidation is complete before proceeding
3. Memory Barriers
Memory barriers are essential to ensure that the cache invalidations and TLB invalidations are completed before the ERET instruction is executed. The DSB (Data Synchronization Barrier) and ISB (Instruction Synchronization Barrier) instructions should be used to enforce the correct ordering of memory operations.
// Example of memory barriers
dsb sy // Ensure all previous memory operations are complete
isb // Ensure the instruction stream is synchronized
4. TLB Invalidation
The kernel must invalidate the TLB entries corresponding to the old memory mappings before switching to the new mappings. This can be done using the TLBI VMALLE1 (TLB Invalidate by VMID, All at stage 1, EL1) instruction, which invalidates all TLB entries for the current VMID at EL1.
// Example of TLB invalidation
tlbi vmalle1 // Invalidate all TLB entries for the current VMID at EL1
dsb sy // Ensure the TLB invalidation is complete
isb // Ensure the instruction stream is synchronized
5. Setting Up ELR and SPSR
Before executing the ERET instruction, the kernel must set up the Exception Link Register (ELR) and Saved Program Status Register (SPSR) to ensure that the processor transitions to EL0 with the correct program counter and processor state.
// Example of setting up ELR and SPSR
mov x0, #0x0 // Set the ELR to the entry point of the user program
msr elr_el1, x0
mov x0, #0x1c0 // Set the SPSR to EL0 with interrupts enabled
msr spsr_el1, x0
6. Executing ERET
Finally, the kernel can execute the ERET instruction to transition to EL0 and begin executing the user program. The processor will use the values in ELR and SPSR to set the program counter and processor state.
// Example of executing ERET
eret // Transition to EL0 and begin executing the user program
7. Verification and Debugging
After implementing the cache invalidation and memory barrier strategy, the kernel should be tested to ensure that the synchronous exceptions no longer occur. If the issue persists, additional debugging may be required to identify any remaining cache coherency or memory synchronization issues. The use of JTAG debugging tools can be invaluable in this process, as they allow for real-time inspection of the processor state and memory contents.
Conclusion
Synchronous exceptions at EL0 during user program execution on ARM Cortex-A processors are often caused by improper handling of cache coherency and memory barriers. By carefully invalidating the data and instruction caches, using memory barriers to enforce the correct ordering of memory operations, and ensuring that the TLB is properly invalidated, the kernel can prevent these exceptions and ensure reliable execution of user-space programs. The steps outlined above provide a comprehensive approach to resolving these issues and should be followed rigorously to achieve a stable and efficient system implementation.
