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. 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 ?
