linux-kernel - Re: [PATCH v8 2/2] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6

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Message-ID: <285cd809-d5e0-4688-ace1-630aa8c30aae@roeck-us.net>
Date: Wed, 14 Feb 2024 17:24:13 -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>, Al Viro <viro@...iv.linux.org.uk>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 2/2] lib: checksum: Use aligned accesses for
 ip_fast_csum and csum_ipv6_magic tests

On 2/14/24 13:41, Charlie Jenkins wrote:
> The test cases for ip_fast_csum and csum_ipv6_magic were failing on a
> variety of architectures that are big endian or do not support
> misalgined accesses. Both of these test cases are changed to support big
> and little endian architectures.
> 
> The test for ip_fast_csum is changed to align the data along (14 +
> NET_IP_ALIGN) bytes which is the alignment of an IP header. The test for
> csum_ipv6_magic aligns the data using a struct. An extra padding field
> is added to the struct to ensure that the size of the struct is the same
> on all architectures (44 bytes).
> 
> The test for csum_ipv6_magic somewhat arbitrarily aligned saddr and
> daddr. This would fail on parisc64 due to the following code snippet in
> arch/parisc/include/asm/checksum.h:
> 
> add		%4, %0, %0\n"
> ldd,ma		8(%1), %6\n"
> ldd,ma		8(%2), %7\n"
> add,dc		%5, %0, %0\n"
> 
> The second add is expecting carry flags from the first add. Normally,
> a double word load (ldd) does not modify the carry flags. However,
> because saddr and daddr may be misaligned, ldd triggers a misalignment
> trap that gets handled in arch/parisc/kernel/unaligned.c. This causes
> many additional instructions to be executed between the two adds. This
> can be easily solved by adding the carry into %0 before executing the
> ldd.
> 
> However, that is not necessary since ipv6 headers should always be
> aligned on a 16-byte boundary on parisc since NET_IP_ALIGN is set to 2
> and the ethernet header size is 14.
> 
> Architectures that set NET_IP_ALIGN to 0 must support misaligned saddr
> and daddr, but that is not tested here.
> 
> Fixes: 6f4c45cbcb00 ("kunit: Add tests for csum_ipv6_magic and ip_fast_csum")
> Signed-off-by: Charlie Jenkins <charlie@...osinc.com>

Reviewed-and-Tested-by: Guenter Roeck <linux@...ck-us.net>

Guenter