Re: [coreboot] cbfs alignment

[Aaron Durbin]

On Fri, Jul 17, 2015 at 2:45 PM, ron minnich  wrote:
> riscv is taking alignment traps reading cbfs.
>
> The issue is that 64-bit fields are 32-bit aligned, which fails many places.
>
> Thaminda found this comment:
> Since coreboot is usually compiled 32bit, gcc will align 64bit
> types to 32bit boundaries. If the coreboot table is dumped on a
> 64bit system, a uint64_t would be aligned to 64bit boundaries,
> breaking the table format.
>
> this is a real problem. Would have broken badly on Alpha, and breaks badly
> on RISCV.
>
> We can fix it, with an ugly macro, but ... what's the right move here?

You mean how none of the structs in src/include/boot/coreboot_tables.h
have a packed attribute decorated on them? And they should be marked
packed since these are a serialized format. That's not really going to
help you.

One thing you do is break up all uint64_t in that header to be a
struct of 2 uint32_t -- which is what lb_uint64 was for I assume. What
that means is the types all need to be marshalled properly, and you'd
have to do this as it is if RISC V doesn't support unaligned accesses.

In general, coreboot has been largely written to assume unaligned
accesses are no big deal (thanks, x86!). Because of this architectures
like ARM *have* to get their MMUs up to enable normal memory so that
it can honor the "handle unaligned accesses".
> ron