Message-ID: <9f756413-806c-4cd0-a6cf-8dd82af14e88@csgroup.eu>
Date: Tue, 27 Feb 2024 18:11:24 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Charlie Jenkins <charlie@...osinc.com>
CC: "Russell King (Oracle)" <linux@...linux.org.uk>, Guenter Roeck
	<linux@...ck-us.net>, David Laight <David.Laight@...lab.com>, Palmer Dabbelt
	<palmer@...belt.com>, Andrew Morton <akpm@...ux-foundation.org>, Helge Deller
	<deller@....de>, "James E.J. Bottomley"
	<James.Bottomley@...senpartnership.com>, Parisc List
	<linux-parisc@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Palmer Dabbelt
	<palmer@...osinc.com>, Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum
 and csum_ipv6_magic tests



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

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