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authorRussell King <rmk@flint.arm.linux.org.uk>2002-02-20 19:38:24 +0000
committerRussell King <rmk@flint.arm.linux.org.uk>2002-02-20 19:38:24 +0000
commita7c2267fc690c9b3ec13e5809957686dd6362f1f (patch)
tree1ec66dab8f109de2e2862a7274d4fcdc290ca986
parent56dca0ab3ab3f87164c8c44147f4e3f2a6891adb (diff)
Documentation for user space ARM alignment trap handler describing
its modes of operation.
-rw-r--r--Documentation/arm/mem_alignment58
1 files changed, 58 insertions, 0 deletions
diff --git a/Documentation/arm/mem_alignment b/Documentation/arm/mem_alignment
new file mode 100644
index 000000000000..a944af7c0244
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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/sys/debug/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 occurences plus the current mode of
+operation for user space code.
+
+
+Nicolas Pitre, Mar 13, 2001. Modified Russell King, Nov 30, 2001.