From: Felix Fietkau <nbd@nbd.name>
To: "Måns Rullgård" <mans@mansr.com>
Cc: linux-mips@linux-mips.org, Netdev <netdev@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
David Miller <davem@davemloft.net>,
WireGuard mailing list <wireguard@lists.zx2c4.com>
Subject: Re: Misalignment, MIPS, and ip_hdr(skb)->version
Date: Sat, 10 Dec 2016 21:36:32 +0100 [thread overview]
Message-ID: <07ae38aa-ffb5-f450-fbbb-001e91d4454d@nbd.name> (raw)
In-Reply-To: <yw1xy3znwmmr.fsf@unicorn.mansr.com>
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
next prev parent reply other threads:[~2016-12-10 20:30 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-07 18:35 Jason A. Donenfeld
2016-12-07 18:47 ` Dave Taht
2016-12-07 18:51 ` David Miller
2016-12-07 18:54 ` Jason A. Donenfeld
2016-12-07 19:52 ` David Miller
2016-12-08 0:29 ` Jason A. Donenfeld
2016-12-08 0:37 ` David Miller
2016-12-08 22:20 ` Jason A. Donenfeld
2016-12-08 23:14 ` David Miller
2016-12-11 8:07 ` Willy Tarreau
2016-12-11 10:47 ` Måns Rullgård
2016-12-10 12:25 ` Felix Fietkau
2016-12-10 13:25 ` Måns Rullgård
2016-12-10 20:09 ` Felix Fietkau
2016-12-10 20:32 ` Måns Rullgård
2016-12-10 20:36 ` Felix Fietkau [this message]
2016-12-12 16:19 ` David Laight
2016-12-12 16:31 ` Måns Rullgård
2016-12-08 0:30 ` Hannes Frederic Sowa
2016-12-08 4:34 ` Daniel Kahn Gillmor
2016-12-09 11:26 ` Jiri Benc
2016-12-10 22:18 ` Dan Lüdtke
2016-12-11 7:15 ` Greg KH
2016-12-11 14:50 ` Jason A. Donenfeld
2016-12-11 15:30 ` Andrew Lunn
2016-12-11 15:37 ` Jason A. Donenfeld
2016-12-11 16:44 ` Willy Tarreau
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