[Bug libc/31492] ARM ldp instruction trigger bus error when kernel open option CONFIG_IO_STRICT_DEVMEM

["adhemerval.zanella at linaro dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31492

Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:

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--- Comment #4 from Adhemerval Zanella <adhemerval.zanella at linaro dot org> ---
This is most likely unaligned access to a device region (due to the
CONFIG_IO_STRICT_DEVMEM), which is not supported by the ISA [1]. Although the
input is aligned, aarch64 memcpy assumes unaligned access;the size of '724'
will trigger the 'copy_long' branch, which copies a multiple of 128 bytes, with
the remaining 84 bytes that it is not aligned to 8 bytes (thus potentially
triggering an unaligned load of the 'copy64_from_end' code path.

We had a similar issue on POWER, which prevented us from adding an unaligned
memcpy optimization as default because memcpy was used in some video drivers on
non-cacheable memory and unaligned VSX operations triggered some bad
performance issues (it is essentially emulated by the kernel). We had to gate
this optimization through a tunable instead [2].

You can raise this to ARM maintainers, but I think it is unlikely that they
will change the default implementation to avoid unaligned access since this is
really a performance improvement for all cases.

[1]
https://developer.arm.com/documentation/102376/0200/Alignment-and-endianness/Alignment
[2] https://sourceware.org/pipermail/libc-alpha/2017-December/089357.html

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