|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
|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
|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/cpu/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.