Le 27/02/2024 à 18:54, Charlie Jenkins a écrit : > 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. I don't have such a list, but I guess you do ? If all platforms have already been fixed, why are you sending this patch at all ? Christophe
