Re: [arm-gnu] Invalid unaligned access support for ARM Cortex-A8 in Sourcery G++ Lite 2011.03-41.

[Carlos O'Donell <carlos@xxxxxxxxxxxxxxxx>]

On 5/15/2011 8:27 AM, Maksim Galemin wrote:
> Hi,
>
> Preconditions:
> Build system: Linux mgalemin-ThinkPad-R51e 2.6.39-1-generic-pae
> #6-Ubuntu SMP Fri May 6 07:11:44 UTC 2011 i686 i686 i386 GNU/Linux
> Cross-compiler: Sourcery G++ Lite 2011.03-41
> Host kernel: 2.6.37-rc7 (from
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git,
> tag "v2.6.39-rc7") with default config for OMAP3 - "omap2plus_defconfig"
> Host hardware: Beagleboard-xM
>
> Execution:
> 1. Checkout Linux kernel (tag "v2.6.39-rc7");
> 2. Make default kernel configuration for OMAP3 using
> "omap2plus_defconfig" target;
> 3. Build kernel using Sourcery G++ Lite 2011.03-41 cross-compiler:
> make CROSS_COMPILE=arm-none-linux-gnueabi- ARCH=arm uImage
>
> Expected results:
> Kernel boots.
>
> Results:
> Kernel crashes with "Unhandled fault: alignment exception" (please see
> attachment for details).
>
> Workaround 1:
> Force structures to be aligned. For example, for exception from the
> attachment change size of "empty_str" array in function
> "pcpu_dump_alloc_info" (file "mm/percpu.c") from:
> char empty_str[] = "--------";
> to:
> char empty_str[16] = "--------";
>
> Workaround 2:
> Compile kernel with disabled "Unaligned access support":
> make CROSS_COMPILE=arm-none-linux-gnueabi- ARCH=arm
> EXTRA_CFLAGS=-mno-unaligned-access uImage

Using unaligned loads is the default for ARMv6, ARMv7-A, ARMv7-R, and 
ARMv7-M, where the hardware handles the unaligned load (the same default 
is used by RealView). Note that hardware support for unaligned loads is 
*considerably* faster than a software unaligned handler.

If you disable the hardware support for unaligned loads (via A-bit/U-bit 
during reset) then you must either provide an unaligned load handler or 
compile with -mno-unaligned-access.

If you find that the compiler generated an invalid unaligned load with 
respect to the architecture standard, then we would need to see a test case.

Cheers,
Carlos.
--
Carlos O'Donell
Mentor Graphics / CodeSourcery
carlos@xxxxxxxxxxxxxxxx
+1 (650) 331-3385 x716