Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752335AbcLJUKJ (ORCPT ); Sat, 10 Dec 2016 15:10:09 -0500 Received: from nbd.name ([46.4.11.11]:45801 "EHLO nbd.name" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751496AbcLJUKH (ORCPT ); Sat, 10 Dec 2016 15:10:07 -0500 Subject: Re: Misalignment, MIPS, and ip_hdr(skb)->version To: =?UTF-8?B?TcOlbnMgUnVsbGfDpXJk?= References: <20161207.135127.789629809982860453.davem@davemloft.net> <040bcdb2-2725-c8de-11d9-a4f77b75d9d8@nbd.name> Cc: "Jason A. Donenfeld" , David Miller , Netdev , WireGuard mailing list , LKML , linux-mips@linux-mips.org From: Felix Fietkau Message-ID: <7f8ba817-73ef-e1e1-4fdf-b9178e922008@nbd.name> Date: Sat, 10 Dec 2016 21:09:57 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0) Gecko/20100101 Thunderbird/45.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1831 Lines: 38 On 2016-12-10 14:25, Måns Rullgård wrote: > Felix Fietkau writes: > >> On 2016-12-07 19:54, Jason A. Donenfeld wrote: >>> On Wed, Dec 7, 2016 at 7:51 PM, David Miller 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