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 11:34:51 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Charlie Jenkins <charlie@...osinc.com>, Guenter Roeck
	<linux@...ck-us.net>, 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 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 ?

I guess we should involve ARM people in this discussion.

Christophe