From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: nbd@nbd.name Received: from krantz.zx2c4.com (localhost [127.0.0.1]) by krantz.zx2c4.com (ZX2C4 Mail Server) with ESMTP id efc1023f for ; Sat, 10 Dec 2016 20:30:45 +0000 (UTC) Received: from nbd.name (nbd.name [46.4.11.11]) by krantz.zx2c4.com (ZX2C4 Mail Server) with ESMTP id 65f09594 for ; Sat, 10 Dec 2016 20:30:45 +0000 (UTC) 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> <7f8ba817-73ef-e1e1-4fdf-b9178e922008@nbd.name> From: Felix Fietkau Message-ID: <07ae38aa-ffb5-f450-fbbb-001e91d4454d@nbd.name> Date: Sat, 10 Dec 2016 21:36:32 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Cc: linux-mips@linux-mips.org, Netdev , LKML , David Miller , WireGuard mailing list List-Id: Development discussion of WireGuard List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 2016-12-10 21:32, Måns Rullgård wrote: > Felix Fietkau writes: > >> 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. > > 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