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 15:17:05 -0800
From: Charlie Jenkins <charlie@...osinc.com>
To: David Laight <David.Laight@...lab.com>
Cc: 'Russell King' <linux@...linux.org.uk>,
	Guenter Roeck <linux@...ck-us.net>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	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 10:33:56PM +0000, David Laight wrote:
> ...
> > I think you misunderstand. "NET_IP_ALIGN offset is what the kernel
> > defines to be supported" is a gross misinterpretation. It is not
> > "defined to be supported" at all. It is the _preferred_ alignment
> > nothing more, nothing less.

This distinction is arbitrary in practice, but I am open to being proven
wrong if you have data to back up this statement. If the driver chooses
to not follow this, then the driver might not work. ARM defines the
NET_IP_ALIGN to be 2 to pad out the header to be on the supported
alignment. If the driver chooses to pad with one byte instead of 2
bytes, the driver may fail to work as the CPU may stall after the
misaligned access.

> 
> I'm sure I've seen code that would realign IP headers to a 4 byte
> boundary before processing them - but that might not have been in
> Linux.
> 
> I'm also sure there are cpu which will fault double length misaligned
> memory transfers - which might be used to marginally speed up code.
> Assuming more than 4 byte alignment for the IP header is likely
> 'wishful thinking'.
> 
> There is plenty of ethernet hardware that can only write frames
> to even boundaries and plenty of cpu that fault misaligned accesses.
> There are even cases of both on the same silicon die.
> 
> You also pretty much never want a fault handler to fixup misaligned
> ethernet frames (or really anything else for that matter).
> It is always going to be better to check in the code itself.
> 
> x86 has just made people 'sloppy' :-)
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

If somebody has a solution they deem to be better, I am happy to change
this test case. Otherwise, I would appreciate a maintainer resolving
this discussion and apply this fix.

- Charlie