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

Felix Fietkau nbd at nbd.name
Sat Dec 10 21:09:57 CET 2016

On 2016-12-10 14:25, Måns Rullgård wrote:
> 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.
I considered that as well, but it's bad for routing performance if the
ethernet MAC does not support scatter/gather for xmit.
Unfortunately that limitation is quite common on embedded hardware.

- Felix

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