On Fri, Nov 20, 2020 at 11:18:34AM +0300, Igor Russkikh wrote: > > > On 20/11/2020 1:49 am, Maciej Fijalkowski wrote: > > External Email > > > > ---------------------------------------------------------------------- > > On Thu, Nov 19, 2020 at 10:34:48PM +0000, Ramsay, Lincoln wrote: > >> When performing IPv6 forwarding, there is an expectation that SKBs > >> will have some headroom. When forwarding a packet from the aquantia > >> driver, this does not always happen, triggering a kernel warning. > >> > >> The build_skb path fails to allow for an SKB header, but the hardware > >> buffer it is built around won't allow for this anyway. Just always use > > the > >> slower codepath that copies memory into an allocated SKB. > >> > >> Signed-off-by: Lincoln Ramsay <lincoln.ram...@opengear.com> > >> --- > > > > (Next time please include in the subject the tree that you're targetting > > the patch) > > > > I feel like it's only a workaround, not a real solution. On previous > > thread Igor says: > > > > "The limitation here is we can't tell HW on granularity less than 1K." > > > > Are you saying that the minimum headroom that we could provide is 1k? > > We can tell HW to place packets with 4 bytes granularity addresses, but the > problem is the length granularity of this buffer - 1K. > > This means we can do as Ramsay initially suggested - just offset the packet > placement. But then we have to guarantee that 1K after this offset is > available to HW.
Ok, I see, thanks for clarifying. > > Since normal layout is 1400 packets - we do use 2K (half page) for each > packet. What is 'normal layout is 1400 packets' ? Didn't you mean the 1500 byte standard MTU? So this is what you've been trying to tell me - that for 1500 byte mtu and 1k HW granularity you need to provide to HW 2k of contiguous space, correct? > This way we reuse each allocated page for at least two packets (and putting > skb_shared into the remaining 512b). I don't think I follow that. I thought that 2k needs to be exclusive for HW and now you're saying that for remaining 512 bytes you can do whatever you want. If that's true then I think you can have build_skb support and I don't see that 1k granularity as a limitation. > > Obviously we may allocate 4K page for a single packet, and tell HW that it can > use 3K for data. This'll give 1K headroom. Quite an overload - assuming IMIX > is of 0.5K - 1.4K.. > > Of course that depends on a usecase. If you know all your traffic is 16K > jumbos - putting 1K headroom is very small overhead on memory usage. > > > Maybe put more pressure on memory side and pull in order-1 pages, provide > > this big headroom and tailroom for skb_shared_info and use build_skb by > > default? With standard 1500 byte MTU. > I know many customers do consider AQC chips in near embedded environments > (routers, etc). They really do care about memories. So that could be risky. We have a knob that is controlled by ethtool's priv flag so you can change the memory model and pull the build_skb out of the picture. Just FYI. > > > This issue would pop up again if this driver would like to support XDP > > where 256 byte headroom will have to be provided. > > Actually it already popped. Thats one of the reasons I'm delaying with xdp > patch series for this driver. > > I think the best tradeoff here would be allocating order 1 or 2 pages (i.e. 8K > or 16K), and reuse the page for multiple placements of 2K XDP packets: > > (256+2048)*3 = 6912 (1K overhead for each 3 packets) > > (256+2048)*7 = 16128 (200b overhead over 7 packets) And for XDP_PASS you would use build_skb? Then tailroom needs to be provided. > > Regards, > Igor > > >
