linux-kernel - RE: [PATCH v9 2/2] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6

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Date: Fri, 23 Feb 2024 10:28:57 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Christophe Leroy' <christophe.leroy@...roup.eu>, Charlie Jenkins
	<charlie@...osinc.com>, Guenter Roeck <linux@...ck-us.net>, 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>, Al Viro <viro@...iv.linux.org.uk>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: RE: [PATCH v9 2/2] lib: checksum: Use aligned accesses for
 ip_fast_csum and csum_ipv6_magic tests

From: Christophe Leroy
> Sent: 23 February 2024 10:07
...
> > +/* Ethernet headers are 14 bytes and NET_IP_ALIGN is used to align them */
> > +#define IP_ALIGNMENT (14 + NET_IP_ALIGN)
> 
> Only if no VLAN.
> 
> When using VLANs it is 4 bytes more. But why do you mind that at all ?

Wasn't one architecture faulting on a double-register read?
Where that had to be aligned (probably 8 bytes) but a normal
memory read could be misaligned?

I doubt it is valid to assume that the IP headers is 8 byte
aligned when NET_IP_ALIGN is 2.

	David

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