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

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Message-ID: <Zdx6YMRdPmb595M2@shell.armlinux.org.uk>
Date: Mon, 26 Feb 2024 11:47:44 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Christophe Leroy <christophe.leroy@...roup.eu>
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

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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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