Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests

[Charlie Jenkins <charlie@rivosinc.com>]

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 ?