Misalignment, MIPS, and ip_hdr(skb)->version

[Måns Rullgård]

Felix Fietkau <nbd at nbd.name> writes:

> On 2016-12-07 19:54, Jason A. Donenfeld wrote:
>> On Wed, Dec 7, 2016 at 7:51 PM, David Miller <davem at davemloft.net> wrote:
>>> It's so much better to analyze properly where the misalignment comes from
>>> and address it at the source, as we have for various cases that trip up
>>> Sparc too.
>> 
>> That's sort of my attitude too, hence starting this thread. Any
>> pointers you have about this would be most welcome, so as not to
>> perpetuate what already seems like an issue in other parts of the
>> stack.
> Hi Jason,
>
> I'm the author of that hackish LEDE/OpenWrt patch that works around the
> misalignment issues. Here's some context regarding that patch:
>
> I intentionally put it in the target specific patches for only one of
> our MIPS targets. There are a few ar71xx devices where the misalignment
> cannot be fixed, because the Ethernet MAC has a 4-byte DMA alignment
> requirement, and does not support inserting 2 bytes of padding to
> correct the IP header misalignment.
>
> With these limitations the choice was between this ugly network stack
> patch or inserting a very expensive memmove in the data path (which is
> better than taking the mis-alignment traps, but still hurts routing
> performance significantly).

I solved this problem in an Ethernet driver by copying the initial part
of the packet to an aligned skb and appending the remainder using
skb_add_rx_frag().  The kernel network stack only cares about the
headers, so the alignment of the packet payload doesn't matter.

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