On Mon, Feb 26, 2024 at 11:57:24AM +0000, Christophe Leroy wrote: > > > 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. Looking again at these two functions, I'm mistaken - this was written for optimal use with 32-bit alignment, not 16-bit. However, the entire IP layer is written with the assumption that for maximum performance, the IP header will be 32-bit aligned. However, that may not always be the case for incoming packets, and what saves 32-bit Arm is the ability to do unaligned loads in later revisions of the architecture, or the alignment fault handler (slow) on older revisions. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!
