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63 lines
2.6 KiB
63 lines
2.6 KiB
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Memory alignment |
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Too many problems popped up because of unnoticed misaligned memory access in |
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kernel code lately. Therefore the alignment fixup is now unconditionally |
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configured in for SA11x0 based targets. According to Alan Cox, this is a |
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bad idea to configure it out, but Russell King has some good reasons for |
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doing so on some f***ed up ARM architectures like the EBSA110. However |
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this is not the case on many design I'm aware of, like all SA11x0 based |
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ones. |
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Of course this is a bad idea to rely on the alignment trap to perform |
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unaligned memory access in general. If those access are predictable, you |
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are better to use the macros provided by include/asm/unaligned.h. The |
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alignment trap can fixup misaligned access for the exception cases, but at |
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a high performance cost. It better be rare. |
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Now for user space applications, it is possible to configure the alignment |
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trap to SIGBUS any code performing unaligned access (good for debugging bad |
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code), or even fixup the access by software like for kernel code. The later |
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mode isn't recommended for performance reasons (just think about the |
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floating point emulation that works about the same way). Fix your code |
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instead! |
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Please note that randomly changing the behaviour without good thought is |
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real bad - it changes the behaviour of all unaligned instructions in user |
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space, and might cause programs to fail unexpectedly. |
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To change the alignment trap behavior, simply echo a number into |
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/proc/cpu/alignment. The number is made up from various bits: |
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=== ======================================================== |
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bit behavior when set |
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=== ======================================================== |
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0 A user process performing an unaligned memory access |
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will cause the kernel to print a message indicating |
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process name, pid, pc, instruction, address, and the |
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fault code. |
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1 The kernel will attempt to fix up the user process |
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performing the unaligned access. This is of course |
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slow (think about the floating point emulator) and |
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not recommended for production use. |
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2 The kernel will send a SIGBUS signal to the user process |
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performing the unaligned access. |
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=== ======================================================== |
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Note that not all combinations are supported - only values 0 through 5. |
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(6 and 7 don't make sense). |
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For example, the following will turn on the warnings, but without |
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fixing up or sending SIGBUS signals:: |
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echo 1 > /proc/cpu/alignment |
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You can also read the content of the same file to get statistical |
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information on unaligned access occurrences plus the current mode of |
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operation for user space code. |
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Nicolas Pitre, Mar 13, 2001. Modified Russell King, Nov 30, 2001.
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