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

["luofengwc at qq dot com" <sourceware-bugzilla@sourceware.org>]

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

--- Comment #5 from luofeng14 <luofengwc at qq dot com> ---
(In reply to Adhemerval Zanella from comment #4)
> 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

Zanella thanks your reply

-- 
You are receiving this mail because:
You are on the CC list for the bug.