Alignment checking on ARM

[Al Grant]

I'm building L4 on ARM using GCC 7.4.0 on Cortex-A15. ARCH-arm/head.cc enables
alignment checking (sets SCTLR.A=1), but we then see an alignment check fault in
init_kip_f.cc. With the default compiler options, the GCC cross tools will assume
alignment checking is _not_ enabled and will generate code that may use unaligned
access.

We can override this by compiling with -mno-unaligned-access. But it would be
good to know what is intended. If the intention is to be able to run "mainstream"
open-source code in userspace, i.e. build code using the standard Linux ABI,
we ought to allow that, and not enable alignment checks in SCTLR. Some code
will not be fixed by compiling -mno-unaligned-access (because it casts arbitrary
unaligned pointers to word-sized types etc.)

So my question is, what is the intention?

  (1) fault unaligned access in kernel and userspace

  (2) fault unaligned access in kernel but allow in userspace

  (3) allow unaligned access in kernel and userspace

I'm assuming it can't be (3) as the bootstrap code explicitly enables alignment checking.
(2) would be a good compromise, it would enable alignment checks which would pick
up some possible faults in the kernel, but it would improve portability of ordinary
open-source packages in userspace.
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