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