Hi,

I just debugged a TF-A boot crash that turned out to be caused by an alignment fault in platform code. Someone had defined some static storage space as a uint8_t array, and then accessed it by dereferencing uint16_t pointers.

Of course this is ultimately a bug in the platform code that should be fixed, but I am still wondering why we choose to set the SCTLR_EL3.A (Alignment fault checking) flag in TF-A? In an ideal world, maybe we could say that code which can generate alignment faults should not exist -- but, unfortunately, people make mistakes, and this kind of mistake may linger unnoticed for a long time in the codebase before randomly getting triggered due to subtle shifts in the binary's memory layout. (Worse, in some situations this could get affected by SMC parameters passed in from lower exception levels, so it would only be noticeable and could possibly be intentionally triggered if the lower exception level passes in just the right values.)

For that reason, most other environments I know (e.g. Linux) always keep that flag cleared. There's no harm in that -- as far as I'm aware all aarch64 cores are required to support unaligned accesses to cached memory types, and the worst that would happen is a slight performance penalty for the access. I think that flag is mostly meant as a debugging feature to be able to shake out accidental unaligned accesses from your code? If our goal is to be stable and reliable firmware, shouldn't we disable it to reduce the chance of unexpected crashes?