linux-libre/Documentation/arm/mem_alignment

Too many problems poped up because of unnoticed misaligned memory access in
kernel code lately.  Therefore the alignment fixup is now unconditionally
configured in for SA11x0 based targets.  According to Alan Cox, this is a
bad idea to configure it out, but Russell King has some good reasons for
doing so on some f***ed up ARM architectures like the EBSA110.  However
this is not the case on many design I'm aware of, like all SA11x0 based
ones.

Of course this is a bad idea to rely on the alignment trap to perform
unaligned memory access in general.  If those access are predictable, you
are better to use the macros provided by include/asm/unaligned.h.  The
alignment trap can fixup misaligned access for the exception cases, but at
a high performance cost.  It better be rare.

Now for user space applications, it is possible to configure the alignment
trap to SIGBUS any code performing unaligned access (good for debugging bad
code), or even fixup the access by software like for kernel code.  The later
mode isn't recommended for performance reasons (just think about the
floating point emulation that works about the same way).  Fix your code
instead!

Please note that randomly changing the behaviour without good thought is
real bad - it changes the behaviour of all unaligned instructions in user
space, and might cause programs to fail unexpectedly.

To change the alignment trap behavior, simply echo a number into
/proc/cpu/alignment.  The number is made up from various bits:

bit		behavior when set
---		-----------------

0		A user process performing an unaligned memory access
		will cause the kernel to print a message indicating
		process name, pid, pc, instruction, address, and the
		fault code.

1		The kernel will attempt to fix up the user process
		performing the unaligned access.  This is of course
		slow (think about the floating point emulator) and
		not recommended for production use.

2		The kernel will send a SIGBUS signal to the user process
		performing the unaligned access.

Note that not all combinations are supported - only values 0 through 5.
(6 and 7 don't make sense).

For example, the following will turn on the warnings, but without
fixing up or sending SIGBUS signals:

	echo 1 > /proc/sys/debug/alignment

You can also read the content of the same file to get statistical
information on unaligned access occurrences plus the current mode of
operation for user space code.


Nicolas Pitre, Mar 13, 2001.  Modified Russell King, Nov 30, 2001.