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Date: Tue, 5 Feb 2019 19:12:16 -0800 From: Alexei Starovoitov <alexei.starovoitov@...il.com> To: Stanislav Fomichev <sdf@...ichev.me> Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, Stanislav Fomichev <sdf@...gle.com>, Network Development <netdev@...r.kernel.org>, David Miller <davem@...emloft.net>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, simon.horman@...ronome.com, Willem de Bruijn <willemb@...gle.com> Subject: Re: [RFC bpf-next 0/7] net: flow_dissector: trigger BPF hook when called from eth_get_headlen On Tue, Feb 05, 2019 at 04:59:31PM -0800, Stanislav Fomichev wrote: > On 02/05, Alexei Starovoitov wrote: > > On Tue, Feb 05, 2019 at 12:40:03PM -0800, Stanislav Fomichev wrote: > > > On 02/05, Willem de Bruijn wrote: > > > > On Tue, Feb 5, 2019 at 12:57 PM Stanislav Fomichev <sdf@...gle.com> wrote: > > > > > > > > > > Currently, when eth_get_headlen calls flow dissector, it doesn't pass any > > > > > skb. Because we use passed skb to lookup associated networking namespace > > > > > to find whether we have a BPF program attached or not, we always use > > > > > C-based flow dissector in this case. > > > > > > > > > > The goal of this patch series is to add new networking namespace argument > > > > > to the eth_get_headlen and make BPF flow dissector programs be able to > > > > > work in the skb-less case. > > > > > > > > > > The series goes like this: > > > > > 1. introduce __init_skb and __init_skb_shinfo; those will be used to > > > > > initialize temporary skb > > > > > 2. introduce skb_net which can be used to get networking namespace > > > > > associated with an skb > > > > > 3. add new optional network namespace argument to __skb_flow_dissect and > > > > > plumb through the callers > > > > > 4. add new __flow_bpf_dissect which constructs temporary on-stack skb > > > > > (using __init_skb) and calls BPF flow dissector program > > > > > > > > The main concern I see with this series is this cost of skb zeroing > > > > for every packet in the device driver receive routine, *independent* > > > > from the real skb allocation and zeroing which will likely happen > > > > later. > > > Yes, plus ~200 bytes on the stack for the callers. > > > > > > Not sure how visible this zeroing though, I can probably try to get some > > > numbers from BPF_PROG_TEST_RUN (running current version vs running with > > > on-stack skb). > > > > imo extra 256 byte memset for every packet is non starter. > We can put pre-allocated/initialized skbs without data into percpu or even > use pcpu_freelist_pop/pcpu_freelist_push to make sure we don't have to think > about having multiple percpu for irq/softirq/process contexts. > Any concerns with that approach? > Any other possible concerns with the overall series? I'm missing why the whole thing is needed. You're saying: " make BPF flow dissector programs be able to work in the skb-less case". What does it mean specifically? The only non-skb case is XDP. Are you saying you want flow_dissector prog to be run in XDP?
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