From: Gregory CLEMENT <gregory.clement@free-electrons.com>
To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Romain Izard <romain.izard.pro@gmail.com>,
Sven Schmidt <4sschmid@informatik.uni-hamburg.de>,
LKML <linux-kernel@vger.kernel.org>,
linux-arm-kernel@lists.infradead.org,
Petr Cvek <petrcvekcz@gmail.com>,
Aaro Koskinen <aaro.koskinen@iki.fi>,
Andrea Adami <andrea.adami@gmail.com>,
Robert Jarzmik <robert.jarzmik@free.fr>
Subject: Re: [PATCH] ARM: add a private asm/unaligned.h
Date: Mon, 30 Oct 2017 14:48:02 +0100 [thread overview]
Message-ID: <87k1zc3fxp.fsf@free-electrons.com> (raw)
In-Reply-To: <20171027152743.GJ20805@n2100.armlinux.org.uk> (Russell King's message of "Fri, 27 Oct 2017 16:27:43 +0100")
Hi Russell,
On ven., oct. 27 2017, Russell King - ARM Linux <linux@armlinux.org.uk> wrote:
> On Fri, Oct 27, 2017 at 05:19:55PM +0200, Gregory CLEMENT wrote:
>> Hi Arnd,
>>
>> On ven., oct. 20 2017, Arnd Bergmann <arnd@arndb.de> wrote:
>>
>> > 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>
>> > ---
>> > Untested so far, please verify that this fixes all the known problems
>> > with the alignment traps.
>>
>> I think Russell already find this conclusion but this patch didn't solve
>> my boot issue with dtb append.
>>
>> I tested this patch onto a v4.14-rc6.
>>
>> Then at least with the patch from Ard: "efi/libstub: arm: omit sorting
>> of the UEFI memory map", it didn't prevent booting.
>
> There's three things wrong, all of which I have patches to address:
>
> 1. The decompressor code reading the image data sometimes issues unaligned
> reads. Some compilers get this wrong and cause an abort. Arnds patch
> addresses this.
>
> 2. Additional sections can appear in the zImage binary which adds extra
> bytes on the end of the image. Concatenating the zImage with the
> extra bytes onto a DTB is the same thing as doing this:
>
> cat zImage extrabytes foo.dtb > image
>
> and the decompressor tolerates no additional bytes between the
> _official_ end of the zImage and the DTB. I've added a patch which
> detects this situation and fails the kernel build when it happens.
So I tested the branch fixes in your git tree.
After doing a "make multi_v7_defconfig; make zImage", I got the message
"arm-linux-gnueabi-ld: error: zImage file size is incorrect" you added
in the commit "ARM: verify size of zImage".
It is the same with mvebu_v7_defconfig, so I wonder wich with
configuration this patch was tested ?
Gregory
>
> 3. Ard's patch "efi/libstub: arm: omit sorting of the UEFI memory map"
> gets rid of the additional sections that (a) change the alignment
> of the compressed data, and (b) add additional unexpected bytes on
> the end of zImage.
>
> So, merely applying Ard's patch will appear to fix the problem, but leave
> the actual underlying issues - so next time we have an additional section
> appear in the zImage, we'd go through all this pain again.
>
> These other patches are required to either make the decompressor tolerant
> or catch the problem cases while they're still in the developer's tree.
>
> --
> RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
> According to speedtest.net: 8.21Mbps down 510kbps up
--
Gregory Clement, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com
next prev parent reply other threads:[~2017-10-30 13:48 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-20 20:01 [PATCH] ARM: add a private asm/unaligned.h Arnd Bergmann
2017-10-20 20:22 ` Ard Biesheuvel
2017-10-23 15:04 ` Romain Izard
2017-10-27 15:19 ` Gregory CLEMENT
2017-10-27 15:27 ` Russell King - ARM Linux
2017-10-30 13:48 ` Gregory CLEMENT [this message]
2017-10-30 14:55 ` Ard Biesheuvel
2017-10-30 15:05 ` Gregory CLEMENT
2017-10-30 15:07 ` Ard Biesheuvel
2017-10-30 15:09 ` Gregory CLEMENT
2017-10-30 15:20 ` Ard Biesheuvel
2017-10-30 15:33 ` Gregory CLEMENT
2017-10-30 15:35 ` Ard Biesheuvel
2017-10-30 15:40 ` Gregory CLEMENT
2017-10-30 16:59 ` Russell King - ARM Linux
2017-10-30 15:55 ` Russell King - ARM Linux
2017-10-30 16:04 ` Gregory CLEMENT
2017-10-30 15:48 ` Russell King - ARM Linux
2017-10-30 16:01 ` Gregory CLEMENT
2017-10-30 16:12 ` Russell King - ARM Linux
2017-10-30 16:24 ` Gregory CLEMENT
2017-10-30 16:38 ` Russell King - ARM Linux
2017-10-31 12:47 ` Russell King - ARM Linux
2017-10-31 12:57 ` Ard Biesheuvel
2017-10-31 13:22 ` Gregory CLEMENT
2017-11-01 15:57 ` Ard Biesheuvel
2017-11-01 18:00 ` Russell King - ARM Linux
2017-11-01 18:02 ` Ard Biesheuvel
2017-11-01 18:11 ` Russell King - ARM Linux
2017-11-01 18:20 ` Ard Biesheuvel
2017-11-01 19:10 ` Russell King - ARM Linux
2017-10-30 15:09 ` Arnd Bergmann
2017-10-30 15:50 ` Russell King - ARM Linux
2017-10-30 17:01 ` Arnd Bergmann
2017-10-30 17:13 ` Russell King - ARM Linux
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