linux-kernel - Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6

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Message-ID: <Zd25XWTkDPuIjpF8@shell.armlinux.org.uk>
Date: Tue, 27 Feb 2024 10:28:45 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Guenter Roeck <linux@...ck-us.net>,
	Charlie Jenkins <charlie@...osinc.com>,
	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

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.

-- 
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