On Tue, Feb 27, 2024 at 06:47:38AM +0000, Christophe Leroy wrote: > > > Le 27/02/2024 à 00:48, Guenter Roeck a écrit : > > On 2/26/24 15:17, Charlie Jenkins wrote: > >> 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. > >> > > Agreed. > > > > I do have a couple of patches which add explicit unaligned tests as well as > > corner case tests (which are intended to trigger as many carry overflows > > as possible). Once I get those working reliably, I'll be happy to submit > > them as additional tests. > > > > The functions definitely have to work at least with and without VLAN, > which means the alignment cannot be greater than 4 bytes. That's also > the outcome of the discussion. Thanks for completely ignoring what I've said. No. The alignment ends up being commonly 2 bytes. As I've said several times, network drivers do _not_ have to respect NET_IP_ALIGN. There are 32-bit ARM drivers which have a DMA engine in them which can only DMA to a 32-bit aligned address. This means that the start of the ethernet header is placed at a 32-bit aligned address making the IP header misaligned to 32-bit. I don't see what is so difficult to understand about this... but it seems that my comments on this are being ignored time and time again, and I can only think that those who are ignoring my comments have some alterior motive here. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!
