Felix Fietkau <nbd@xxxxxxxx> writes: > On 2016-12-07 19:54, Jason A. Donenfeld wrote: >> On Wed, Dec 7, 2016 at 7:51 PM, David Miller <davem@xxxxxxxxxxxxx> 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. -- Måns Rullgård
