linux/Documentation/arm/mem_alignment
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   1Too many problems poped up because of unnoticed misaligned memory access in
   2kernel code lately.  Therefore the alignment fixup is now unconditionally
   3configured in for SA11x0 based targets.  According to Alan Cox, this is a
   4bad idea to configure it out, but Russell King has some good reasons for
   5doing so on some f***ed up ARM architectures like the EBSA110.  However
   6this is not the case on many design I'm aware of, like all SA11x0 based
   7ones.
   8
   9Of course this is a bad idea to rely on the alignment trap to perform
  10unaligned memory access in general.  If those access are predictable, you
  11are better to use the macros provided by include/asm/unaligned.h.  The
  12alignment trap can fixup misaligned access for the exception cases, but at
  13a high performance cost.  It better be rare.
  14
  15Now for user space applications, it is possible to configure the alignment
  16trap to SIGBUS any code performing unaligned access (good for debugging bad
  17code), or even fixup the access by software like for kernel code.  The later
  18mode isn't recommended for performance reasons (just think about the
  19floating point emulation that works about the same way).  Fix your code
  20instead!
  21
  22Please note that randomly changing the behaviour without good thought is
  23real bad - it changes the behaviour of all unaligned instructions in user
  24space, and might cause programs to fail unexpectedly.
  25
  26To change the alignment trap behavior, simply echo a number into
  27/proc/cpu/alignment.  The number is made up from various bits:
  28
  29bit             behavior when set
  30---             -----------------
  31
  320               A user process performing an unaligned memory access
  33                will cause the kernel to print a message indicating
  34                process name, pid, pc, instruction, address, and the
  35                fault code.
  36
  371               The kernel will attempt to fix up the user process
  38                performing the unaligned access.  This is of course
  39                slow (think about the floating point emulator) and
  40                not recommended for production use.
  41
  422               The kernel will send a SIGBUS signal to the user process
  43                performing the unaligned access.
  44
  45Note that not all combinations are supported - only values 0 through 5.
  46(6 and 7 don't make sense).
  47
  48For example, the following will turn on the warnings, but without
  49fixing up or sending SIGBUS signals:
  50
  51        echo 1 > /proc/sys/debug/alignment
  52
  53You can also read the content of the same file to get statistical
  54information on unaligned access occurrences plus the current mode of
  55operation for user space code.
  56
  57
  58Nicolas Pitre, Mar 13, 2001.  Modified Russell King, Nov 30, 2001.
  59
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