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diff --git a/Documentation/arm/mem_alignment.rst b/Documentation/arm/mem_alignment.rst new file mode 100644 index 000000000..aa22893b6 --- /dev/null +++ b/Documentation/arm/mem_alignment.rst @@ -0,0 +1,63 @@ +================ +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. |