Hello,
I'm using btrfs on am ARMv7 and it turns out, that the kernel has to
fixup a lot of kernel originated alignment issues.
See /proc/cpu/alignment (~4h of uptime):
> System: 22304815 (btrfs_get_token_64+0x13c/0x148 [btrfs])
For example, when compiling the kernel on a btrfs volume the counter
increases by 100...1000 per second.
The function shown "btrfs_get_token_64()" is defined here:
> http://lxr.free-electrons.com/source/fs/btrfs/struct-funcs.c#L53
...it already uses get_unaligned_leXX accessors.
Quoting a comment in arch/arm/mm/alignment.c:
* ARMv6 and later CPUs can perform unaligned accesses for
* most single load and store instructions up to word size.
* LDM, STM, LDRD and STRD still need to be handled.
But on a 32bit ARMv7 64bits are not word-sized.
Is the exception and fixup overhead neglectable? Do we have to introduce
something like HAVE_EFFICIENT_UNALIGNED_64BIT_ACCESS?
regards,
Marc
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