On Tue, Feb 27, 2024 at 11:32:19AM +0000, Christophe Leroy wrote: > > > Le 27/02/2024 à 11:28, Russell King (Oracle) a écrit : > > 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. > > > > I'm sorry for this misunderstanding. I'm not ignoring what you said at > all. I understood that ARM is able to handle unaligned accesses with > some exception handlers at worst case and that DMA constraints may lead > to the IP header beeing on a 2 bytes alignment only. > > However I also understood from others that some architectures can't > handle such a 2 bytes only alignments. > > It's been suggested during the discussion that alignment tests should be > added later in a follow-up patch. So for the time being I'm trying to > find a compromise and get the existing tests working on all platforms > but with a smaller alignment than the 16-bytes alignment brought by > Charlie's v10 patch. And a 4 bytes alignment seemed to me to be a good > compromise for this fix. The idea is also to make the fix as minimal as > possible, unlike Charlie's patch that is churning up the tests quite > heavily. Do you have a list of platforms this is failing on? I haven't seen any reports that haven't been fixed. - Charlie > > But maybe I misunderstood some of the discussion and indeed 2 bytes > alignment would work on all platforms and only an odd alignment is > problematic ?
