linux-kernel - Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6

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Date: Mon, 26 Feb 2024 08:44:29 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
 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>, Arnd Bergmann <arnd@...db.de>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Palmer Dabbelt <palmer@...osinc.com>,
 Linux ARM <linux-arm-kernel@...ts.infradead.org>,
 Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum
 and csum_ipv6_magic tests

On 2/26/24 03:34, Christophe Leroy wrote:
> 
> 
> Le 23/02/2024 à 23:11, Charlie Jenkins a écrit :
>> 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.
> 
> I'm still wondering what we are really trying to fix here.
> 
> All other tests are explicitely testing that it works with any alignment.
> 
> Shouldn't ip_fast_csum() and csum_ipv6_magic() work for any alignment as
> well ? I would expect it, I see no comment in arm code which explicits
> that assumption around those functions.
> 
> Isn't the problem only the following line, because csum_offset is
> unaligned ?
> 
> csum = *(__wsum *)(random_buf + i + csum_offset);
> 
> Otherwise, if there really is an alignment issue for the IPv6 source or
> destination address, isn't it enough to perform a 32 bits alignment ?
> 

It isn't just arm.

Question should be what alignments the functions are supposed to be able
to handle, not what they are optimized for. If byte and/or half word alignments
are expected to be supported, there is still architecture code which would
have to be fixed. Unaligned accesses are known to fail on hppa64/parisc64
and on sh4, for example. If unaligned accesses are expected to be handled,
it would probably make sense to add a separate test case, though, to clarify
that the test fails due to alignment issues, not due to input parameters.

Thanks,
Guenter