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

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On 2016-12-10 21:32, Måns Rullgård wrote:
> Felix Fietkau <nbd@nbd.name> writes:
> 
>> On 2016-12-10 14:25, Måns Rullgård wrote:
>>> Felix Fietkau <nbd@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@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.
> 
> Yes, I can see that being an issue.  However, if you're doing zero-copy
> routing, the header part of the original buffer should still be there,
> unused, so you could presumably copy the header of the outgoing packet
> there and then do dma as usual.  Maybe there's something in the network
> stack that makes this impossible though.
That still puts more pressure on the ridiculously small dcache sizes
that are typical for embedded MIPS routers.

- Felix

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Thread

Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-07 19:40 +0100
  Re: Misalignment, MIPS, and ip_hdr(skb)->version Dave Taht <dave.taht@gmail.com> - 2016-12-07 19:50 +0100
    Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-07 20:00 +0100
      Re: Misalignment, MIPS, and ip_hdr(skb)->version David Miller <davem@davemloft.net> - 2016-12-07 21:00 +0100
        Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-08 01:40 +0100
          Re: Misalignment, MIPS, and ip_hdr(skb)->version David Miller <davem@davemloft.net> - 2016-12-08 01:40 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-08 23:30 +0100
              Re: Misalignment, MIPS, and ip_hdr(skb)->version David Miller <davem@davemloft.net> - 2016-12-09 00:20 +0100
              Re: Misalignment, MIPS, and ip_hdr(skb)->version Willy Tarreau <w@1wt.eu> - 2016-12-11 09:10 +0100
                Re: Misalignment, MIPS, and ip_hdr(skb)->version Måns Rullgård <mans@mansr.com> - 2016-12-11 11:50 +0100
      Re: Misalignment, MIPS, and ip_hdr(skb)->version Felix Fietkau <nbd@nbd.name> - 2016-12-10 13:30 +0100
        Re: Misalignment, MIPS, and ip_hdr(skb)->version Måns Rullgård <mans@mansr.com> - 2016-12-10 14:30 +0100
          Re: Misalignment, MIPS, and ip_hdr(skb)->version Felix Fietkau <nbd@nbd.name> - 2016-12-10 21:20 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Felix Fietkau <nbd@nbd.name> - 2016-12-10 21:40 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Måns Rullgård <mans@mansr.com> - 2016-12-10 21:40 +0100
          RE: Misalignment, MIPS, and ip_hdr(skb)->version David Laight <David.Laight@ACULAB.COM> - 2016-12-12 17:20 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Måns Rullgård <mans@mansr.com> - 2016-12-12 17:40 +0100
    Re: Misalignment, MIPS, and ip_hdr(skb)->version David Miller <davem@davemloft.net> - 2016-12-07 20:00 +0100
  Re: Misalignment, MIPS, and ip_hdr(skb)->version Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-12-08 01:40 +0100
    Re: Misalignment, MIPS, and ip_hdr(skb)->version Daniel Kahn Gillmor <dkg@fifthhorseman.net> - 2016-12-08 06:10 +0100
      Re: Misalignment, MIPS, and ip_hdr(skb)->version Jiri Benc <jbenc@redhat.com> - 2016-12-09 12:30 +0100
      Re: Misalignment, MIPS, and ip_hdr(skb)->version Dan Lüdtke <mail@danrl.com> - 2016-12-10 23:30 +0100
        Re: Misalignment, MIPS, and ip_hdr(skb)->version Greg KH <gregkh@linuxfoundation.org> - 2016-12-11 08:20 +0100
          Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-11 16:00 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-11 16:40 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Andrew Lunn <andrew@lunn.ch> - 2016-12-11 16:40 +0100
            Re: Misalignment, MIPS, and ip_hdr(skb)->version Willy Tarreau <w@1wt.eu> - 2016-12-11 17:50 +0100

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