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Groups > linux.kernel > #1727794
| From | Arnd Bergmann <arnd@arndb.de> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) |
| Date | 2017-09-07 00:40 +0200 |
| Message-ID | <umR9f-81I-9@gated-at.bofh.it> (permalink) |
| References | <um2gq-7HU-13@gated-at.bofh.it> <umPAw-6Vc-25@gated-at.bofh.it> <umQZA-7Yw-7@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
On Thu, Sep 7, 2017 at 12:23 AM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> On 6 September 2017 at 21:57, Arnd Bergmann <arnd@arndb.de> wrote:
>> On Mon, Sep 4, 2017 at 6:19 PM, Romain Izard <romain.izard.pro@gmail.com> wrote:
>
> HAVE_EFFICIENT_UNALIGNED_ACCESS only affects explicit unaligned
> accesses, and selects between fixups in hardware or in software.
> AFAICT the issue here is implicit unaligned accesses, where char
> pointers are passed as u32 * arguments.
The problem with include/linux/unaligned/access_ok.h is that it
converts pointers
that are known by the caller to be potentially unaligned and accesses them as if
they were aligned. This means we require a software fixup through the
trap handler
on ARM in cases that the compiler already knows how to handle correctly when
using linux/unaligned/le_struct.h. On ARMv7 this means it ends up using normal
load/store instructures but not the ldm/stm or ldrd/stdr instructions
that are not
allowed on unaligned pointers.
Doing that solves the problem that Romain ran into and also makes other
code much more efficient on ARMv7.
Arnd
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HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) Romain Izard <romain.izard.pro@gmail.com> - 2017-09-04 18:20 +0200
Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) Arnd Bergmann <arnd@arndb.de> - 2017-09-06 23:00 +0200
Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-09-07 00:30 +0200
Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) Arnd Bergmann <arnd@arndb.de> - 2017-09-07 00:40 +0200
Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-09-07 00:50 +0200
Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) Arnd Bergmann <arnd@arndb.de> - 2017-09-07 01:20 +0200
Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-09-07 01:40 +0200
Re: HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32 (was: Alignment issues in zImage with Linux 4.12, LZ4 and GCC5.3) Arnd Bergmann <arnd@arndb.de> - 2017-09-08 22:10 +0200
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