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

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Message-ID: <96b3fcfd-6932-4987-9831-5abdad8d445c@csgroup.eu>
Date: Mon, 26 Feb 2024 11:57:24 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
CC: 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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Palmer Dabbelt
	<palmer@...osinc.com>, Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum
 and csum_ipv6_magic tests



Le 26/02/2024 à 12:47, Russell King (Oracle) a écrit :
> On Mon, Feb 26, 2024 at 11:34:51AM +0000, 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.
> 
> No, these functions are explicitly *not* designed to be used with any
> alignment. They are for 16-bit alignment only.
> 
> I'm not sure where the idea that "any alignment" has come from, but it's
> never been the case AFAIK that we've supported that - or if we do now,
> that's something which has crept in under the radar.
> 

Ok, 16-bit is fine for me, then there is no need to require a (14 + 
NET_IP_ALIGN) ie a 16-bytes (128-bit) alignment as this patch is doing.

Christophe