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

[Charlie Jenkins <charlie@rivosinc.com>]

On Tue, Feb 27, 2024 at 10:28:45AM +0000, Russell King (Oracle) wrote:
> 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!

I don't understand how the capabilities of some ARM drivers factor in
here. It appears that a common case for calling this function is to pass
in an IP header that is aligned along an ethernet header + NET_IP_ALIGN.
It is perfectly acceptable that some drivers don't align along
NET_IP_ALIGN, but that does not seem relevant here.

This test case is supposed to be as true to the "general case" as
possible, so I have aligned the data along 14 + NET_IP_ALIGN. On ARM
this will be a 16-byte boundary since NET_IP_ALIGN is 2. A driver that
does not follow this may not be appropriately tested by this test case,
but anyone is welcome to submit additional test cases that address this
additional alignment concern.

- Charlie