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- ================
- Memory alignment
- ================
- Too many problems popped 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/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.
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