| Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1 | Too many problems poped up because of unnoticed misaligned memory access in |
| 2 | kernel code lately. Therefore the alignment fixup is now unconditionally |
| 3 | configured in for SA11x0 based targets. According to Alan Cox, this is a |
| 4 | bad idea to configure it out, but Russell King has some good reasons for |
| 5 | doing so on some f***ed up ARM architectures like the EBSA110. However |
| 6 | this is not the case on many design I'm aware of, like all SA11x0 based |
| 7 | ones. |
| 8 | |
| 9 | Of course this is a bad idea to rely on the alignment trap to perform |
| 10 | unaligned memory access in general. If those access are predictable, you |
| 11 | are better to use the macros provided by include/asm/unaligned.h. The |
| 12 | alignment trap can fixup misaligned access for the exception cases, but at |
| 13 | a high performance cost. It better be rare. |
| 14 | |
| 15 | Now for user space applications, it is possible to configure the alignment |
| 16 | trap to SIGBUS any code performing unaligned access (good for debugging bad |
| 17 | code), or even fixup the access by software like for kernel code. The later |
| 18 | mode isn't recommended for performance reasons (just think about the |
| 19 | floating point emulation that works about the same way). Fix your code |
| 20 | instead! |
| 21 | |
| 22 | Please note that randomly changing the behaviour without good thought is |
| 23 | real bad - it changes the behaviour of all unaligned instructions in user |
| 24 | space, and might cause programs to fail unexpectedly. |
| 25 | |
| 26 | To change the alignment trap behavior, simply echo a number into |
| 27 | /proc/sys/debug/alignment. The number is made up from various bits: |
| 28 | |
| 29 | bit behavior when set |
| 30 | --- ----------------- |
| 31 | |
| 32 | 0 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 | |
| 37 | 1 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 | |
| 42 | 2 The kernel will send a SIGBUS signal to the user process |
| 43 | performing the unaligned access. |
| 44 | |
| 45 | Note that not all combinations are supported - only values 0 through 5. |
| 46 | (6 and 7 don't make sense). |
| 47 | |
| 48 | For example, the following will turn on the warnings, but without |
| 49 | fixing up or sending SIGBUS signals: |
| 50 | |
| 51 | echo 1 > /proc/sys/debug/alignment |
| 52 | |
| 53 | You can also read the content of the same file to get statistical |
| 54 | information on unaligned access occurrences plus the current mode of |
| 55 | operation for user space code. |
| 56 | |
| 57 | |
| 58 | Nicolas Pitre, Mar 13, 2001. Modified Russell King, Nov 30, 2001. |