Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Charlie Jenkins <charlie@rivosinc.com>,
	Guenter Roeck <linux@roeck-us.net>,
	David Laight <David.Laight@aculab.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Helge Deller <deller@gmx.de>,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	Parisc List <linux-parisc@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Palmer Dabbelt <palmer@rivosinc.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests
Date: Mon, 26 Feb 2024 11:57:24 +0000	[thread overview]
Message-ID: <96b3fcfd-6932-4987-9831-5abdad8d445c@csgroup.eu> (raw)
In-Reply-To: <Zdx6YMRdPmb595M2@shell.armlinux.org.uk>

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