From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Guenter Roeck <linux@roeck-us.net>,
Charlie Jenkins <charlie@rivosinc.com>,
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
Date: Tue, 27 Feb 2024 10:28:45 +0000 [thread overview]
Message-ID: <Zd25XWTkDPuIjpF8@shell.armlinux.org.uk> (raw)
In-Reply-To: <9b4ce664-3ddb-4789-9d5d-8824f9089c48@csgroup.eu>
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.
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!
next prev parent reply other threads:[~2024-02-27 10:29 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-23 22:11 [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests Charlie Jenkins
2024-02-25 15:58 ` Guenter Roeck
2024-02-26 11:34 ` Christophe Leroy
2024-02-26 11:47 ` Russell King (Oracle)
2024-02-26 11:57 ` Christophe Leroy
2024-02-26 12:03 ` Russell King (Oracle)
2024-02-26 16:44 ` Guenter Roeck
2024-02-26 17:50 ` Russell King (Oracle)
2024-02-26 18:35 ` Charlie Jenkins
2024-02-26 19:06 ` Russell King (Oracle)
2024-02-26 19:19 ` Charlie Jenkins
2024-02-26 22:33 ` David Laight
2024-02-26 23:17 ` Charlie Jenkins
2024-02-26 23:48 ` Guenter Roeck
2024-02-27 6:47 ` Christophe Leroy
2024-02-27 10:28 ` Russell King (Oracle) [this message]
2024-02-27 11:32 ` Christophe Leroy
2024-02-27 17:54 ` Charlie Jenkins
2024-02-27 18:11 ` Christophe Leroy
2024-02-27 18:21 ` Charlie Jenkins
2024-02-27 18:35 ` Christophe Leroy
2024-02-27 19:04 ` Charlie Jenkins
2024-02-27 19:31 ` Guenter Roeck
2024-02-27 22:44 ` David Laight
2024-02-28 5:19 ` Guenter Roeck
2024-02-28 0:24 ` Charlie Jenkins
2024-02-28 0:21 ` Charlie Jenkins
2024-02-28 7:25 ` Christophe Leroy
2024-02-28 7:59 ` Guenter Roeck
2024-02-28 10:15 ` Geert Uytterhoeven
2024-02-28 15:40 ` Guenter Roeck
2024-02-29 8:07 ` David Gow
2024-02-29 19:38 ` Charlie Jenkins
2024-02-29 20:22 ` Guenter Roeck
2024-03-01 7:00 ` Christophe Leroy
2024-03-01 6:46 ` Christophe Leroy
2024-03-01 16:24 ` Guenter Roeck
2024-03-01 20:47 ` Guenter Roeck
2024-02-27 11:17 ` Geert Uytterhoeven
2024-02-27 17:55 ` Charlie Jenkins
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