64 lines
2.6 KiB
ReStructuredText
64 lines
2.6 KiB
ReStructuredText
================
|
|
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.
|