Add the rt linux 4.1.3-rt3 as base

[kvmfornfv.git] / kernel / Documentation / arm / mem_alignment

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+Too many problems poped 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/sys/debug/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.