On Tue, Feb 05, 2019 at 07:56:19PM -0800, Stanislav Fomichev wrote: > On 02/05, Alexei Starovoitov wrote: > > 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 > > > > > > <s...@google.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? > eth_get_headlen that drivers call on RX path on a chunk of data to > guesstimate the length of the headers calls flow dissector without an skb > (__skb_flow_dissect was a weird interface where it accepts skb or > data+len). Right now, there is no way to trigger BPF flow dissector > for this case (we don't have an skb to get associated namespace/etc/etc). > The patch series tries to fix that to make sure that we always trigger > BPF program if it's attached to a device's namespace.
then why not to create flow_dissector prog type that works without skb? Why do you need to fake an skb? XDP progs work just fine without it.
