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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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 ?