linux-kernel - Re: [PATCH v11] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6

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Date: Fri, 1 Mar 2024 10:32:36 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Charlie Jenkins <charlie@...osinc.com>,
 David Laight <David.Laight@...lab.com>, Palmer Dabbelt <palmer@...belt.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Helge Deller <deller@....de>,
 "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
 Parisc List <linux-parisc@...r.kernel.org>,
 Christophe Leroy <christophe.leroy@...roup.eu>, Arnd Bergmann
 <arnd@...db.de>, Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: linux-kernel@...r.kernel.org, Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: [PATCH v11] lib: checksum: Use aligned accesses for ip_fast_csum
 and csum_ipv6_magic tests

On 2/29/24 14:46, Charlie Jenkins wrote:
> The test cases for ip_fast_csum and csum_ipv6_magic were not properly
> aligning the IP header, which were causing failures on architectures
> that do not support misaligned accesses like some ARM platforms. To
> solve this, align the data along (14 + NET_IP_ALIGN) bytes which is the
> standard alignment of an IP header and must be supported by the
> architecture.
> 
> Furthermore, all architectures except the m68k pad "struct
> csum_ipv6_magic_data" to 44 bytes. To make compatible with the m68k,
> manually pad this structure to 44 bytes.
> 
> Fixes: 6f4c45cbcb00 ("kunit: Add tests for csum_ipv6_magic and ip_fast_csum")
> Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> Reviewed-by: Guenter Roeck <linux@...ck-us.net>
> Acked-by: Palmer Dabbelt <palmer@...osinc.com>
> ---
> The ip_fast_csum and csum_ipv6_magic tests did not work on all
> architectures due to differences in misaligned access support.
> Fix those issues by changing endianness of data and aligning the data.
> 
> This patch relies upon a patch from Christophe:
> 
> [PATCH net] kunit: Fix again checksum tests on big endian CPUs
> 

Various test results:

On v6.8-rc6-120-g87adedeba51a (current mainline), without this patch

- mps2-an385:mps2_defconfig crashes in IPv6 checksum tests
- ipv6 checksum tests fail on parisc, parisc64, sh, and sheb.

The previously seen problems on big endian systems are still seen with
v6.8-rc6, but are gone after commit 3d6423ef8d51 ("kunit: Fix again
checksum tests on big endian CPUs") has been applied upstream. This includes
the test failures seen with m68k.

The parisc/parisc64 test failures are independent of this patch. Fixes are
available in linux-next and pending in qemu. The sh/sheb failures are due
to upstream commit cadc4e1a2b4 and are no longer seen after reverting that
patch.

This leaves the mps2-an385:mps2_defconfig crash, which is avoided by this patch.
My understanding, which may be wrong, is that arm images with thumb instructions
do not support unaligned accesses (maybe I should say do not support unaligned
accesses with the mps2-an385 qemu emulation; I did not test with real hardware,
after all).

Given all that, the continued discussion around the subject, and the lack
of agreement if unaligned accesses should be tested or not, I don't really
see a path forward for this patch. The remaining known problem is arm with
thumb instructions. I don't think that is going to be fixed. I suspect that
no one but me even tries to run that code (or any arm:nommu images, for that
matter). I'd suggest to drop this patch, and I'll stop testing IP checksum
generation for mps2-an385:mps2_defconfig.

Sorry for all the noise this has generated.

Thanks,
Guenter