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

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Message-ID: <0bd79258-b8c4-45f9-9201-4d7ddf02dfea@csgroup.eu>
Date: Fri, 1 Mar 2024 06:46:05 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Guenter Roeck <linux@...ck-us.net>, 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



Le 26/02/2024 à 17:44, Guenter Roeck a écrit :
> 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.
> 

When you say "Unaligned accesses are known to fail on hppa64/parisc64 
and on sh4", do you mean unaligned accesses in general or do you mean 
ip_fast_csum() with unaligned ip header and csum_ipv6_magic() with 
unaligned source and dest addresses ?

Because later in this thread it is said that only ARM and NIOS2 
potentially have an issue.

And when you say "unaligned", to what level is that ? Is it 4-bytes 
alignment or more or less ?

Christophe