Messages in this thread | | | From | Romain Izard <> | Date | Mon, 23 Oct 2017 17:04:46 +0200 | Subject | Re: [PATCH] ARM: add a private asm/unaligned.h |
| |
2017-10-20 22:01 GMT+02:00 Arnd Bergmann <arnd@arndb.de>: > The asm-generic/unaligned.h header provides two different implementations > for accessing unaligned variables: the access_ok.h version used when > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set pretends that all pointers > are in fact aligned, while the le_struct.h version convinces gcc that the > alignment of a pointer is '1', to make it issue the correct load/store > instructions depending on the architecture flags. > > On ARMv5 and older, we always use the second version, to let the compiler > use byte accesses. On ARMv6 and newer, we currently use the access_ok.h > version, so the compiler can use any instruction including stm/ldm and > ldrd/strd that will cause an alignment trap. This trap can significantly > impact performance when we have to do a lot of fixups and, worse, has > led to crashes in the LZ4 decompressor code that does not have a trap > handler. > > This adds an ARM specific version of asm/unaligned.h that uses the > le_struct.h/be_struct.h implementation unconditionally. This should lead > to essentially the same code on ARMv6+ as before, with the exception of > using regular load/store instructions instead of the trapping instructions > multi-register variants. > > The crash in the LZ4 decompressor code was probably introduced by the > patch replacing the LZ4 implementation, commit 4e1a33b105dd ("lib: update > LZ4 compressor module"), so linux-4.11 and higher would be affected most. > However, we probably want to have this backported to all older stable > kernels as well, to help with the performance issues. > > There are two follow-ups that I think we should also work on, but not > backport to stable kernels, first to change the asm-generic version of > the header to remove the ARM special case, and second to review all > other uses of CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to see if they > might be affected by the same problem on ARM. > > Cc: stable@vger.kernel.org > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Romain.Izard <romain.izard.pro@gmail.com>
Without this patch, the LZ4 decompression code is invalid in v4.14-rc6, when compiled with arm-linux-gnueabihf-gcc, version 7.2. The zImage does not start.
With this patch, the decompression code is valid, and the kernel boots without a problem. No fixups are reported in /proc/cpu/alignment.
-- Romain Izard
| |