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

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
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 11:47:44 +0000	[thread overview]
Message-ID: <Zdx6YMRdPmb595M2@shell.armlinux.org.uk> (raw)
In-Reply-To: <7ae930a7-3b10-4470-94ee-89cb650b3349@csgroup.eu>

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.

-- 
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