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Date: Tue, 27 Feb 2024 10:21:34 -0800
From: Charlie Jenkins <charlie@rivosinc.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>,
	Guenter Roeck <linux@roeck-us.net>,
	David Laight <David.Laight@aculab.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Helge Deller <deller@gmx.de>,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	Parisc List <linux-parisc@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Palmer Dabbelt <palmer@rivosinc.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum
 and csum_ipv6_magic tests
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On Tue, Feb 27, 2024 at 06:11:24PM +0000, Christophe Leroy wrote:
> 
> 
> Le 27/02/2024 ? 18:54, Charlie Jenkins a ?crit?:
> > On Tue, Feb 27, 2024 at 11:32:19AM +0000, Christophe Leroy wrote:
> >>
> >>
> >> Le 27/02/2024 ? 11:28, Russell King (Oracle) a ?crit?:
> >>> On Tue, Feb 27, 2024 at 06:47:38AM +0000, Christophe Leroy wrote:
> >>>>
> >>>>
> >>>> Le 27/02/2024 ? 00:48, Guenter Roeck a ?crit?:
> >>>>> 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 ?

This patch is what is doing the "fixing". Over the course of 10 versions
I have "fixed" the test cases to work on platforms that have various
alignment and endianness constraints. The endianness changes were picked
off of these patches and spun out into a different patch by you. 

I originally introduced these two new test cases since I wrote the riscv
checksum function implementations and these tests were helpful for me
and I figured they may be helpful for somebody else too.

- Charlie

> 
> Christophe