[CPU-863] Misaligned Memory Access May Lead to Exception

Affected revisions: v0.0 v0.1 v0.2

Description

A misaligned load instruction immediately followed by a store instruction may raise a load access fault exception (exception code 5) when the load address and the store address lie in regions with different access permissions configured via Physical Memory Protection (PMP) or Physical Memory Attributes (PMA).

For example, the following sequence commonly triggers this issue when copying an unaligned word from flash or ROM into RAM:

lw  a0, 0(a1)   # unaligned 32-bit address load from a read-only PMP region (e.g., ROM)
sw  a0, 0(a2)   # store to a read-write PMP region (e.g., RAM)

Root cause:

A misaligned load that crosses a word boundary is split internally into two transactions. Due to a timing alignment issue in the design, the second transaction of the misaligned load can incorrectly be checked against the store’s access permissions instead of its own, triggering an exception.

Workarounds

The RISC-V specification does not guarantee that hardware supports misaligned accesses. General-purpose RISC-V code should not rely on that support. If misaligned accesses cannot be avoided, such as when porting third-party libraries, use the following workarounds:

  • Insert one or more nop instructions between the misaligned load and the following store. This prevents the incorrect permission check described above.

    Note

    Linkers that support the --fix-esp-pmp-misalign option can insert nop instructions between consecutive load and store instructions automatically. This option is conservative, as it may also insert extra nop instructions for pairs that would not trigger this exception, for example, when the accesses are aligned or when both access the same PMP region.

  • When copying memory, use misaligned load/store only for the first and last misaligned words of a data block, and use aligned load/store for the words in between. This approach requires fewer nop instructions and has better performance than inserting nop instructions throughout.

  • Configure the same PMP/PMA permissions for the memory regions accessed by the misaligned load and the following store. This may require saving and restoring the permission settings around the affected code.

Solution

No fix scheduled.