On 2016-12-10 21:32, Måns Rullgård wrote: > Felix Fietkau <nbd@xxxxxxxx> writes: > >> On 2016-12-10 14:25, Måns Rullgård wrote: >>> 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. >> >> 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
