netdev - Re: [RFC bpf-next v3 6/8] flow_dissector: handle no-skb use case

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Date:   Tue, 26 Mar 2019 10:48:03 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, davem@...emloft.net, ast@...nel.org,
        daniel@...earbox.net, simon.horman@...ronome.com,
        willemb@...gle.com, peterpenkov96@...il.com
Subject: Re: [RFC bpf-next v3 6/8] flow_dissector: handle no-skb use case

On Tue, Mar 26, 2019 at 09:45:29AM -0700, Stanislav Fomichev wrote:
> On 03/25, Alexei Starovoitov wrote:
> > On Sat, Mar 23, 2019 at 09:05:31AM -0700, Stanislav Fomichev wrote:
> > > On 03/22, Alexei Starovoitov wrote:
> > > > On Fri, Mar 22, 2019 at 06:19:57PM -0700, Stanislav Fomichev wrote:
> > > > > Are we ok with breaking api in this case? I'm all in on removing this
> > > > > extra information. We can always put it back if somebody complains (and
> > > > > manually parse in eth_get_headlen case).
> > > > 
> > > > Fine. That seems to be the only way forward to clean it all up.
> > > > Could you submit patch 1 to bpf tree disallowing vlan fields?
> > > > Patch 3 looks like candidate as well?
> > > SGTM, will do. Let me also spend some time and do a simple test for
> > > the vlan case, to make sure I didn't miss something important.
> > > One question here though: would I need to wait for bpf and bpf-next
> > > to re-merge to continues the series? Or we can cherry-pick those
> > > patches to bpf-next as well (and git will work it out during the
> > > merge)?
> > > 
> > > > > We can still have protocol, because in both skb/skb-less cases we have
> > > > > it.
> > > > 
> > > > proto can work in both cases, but is it needed ? Does program benefit from it?
> > > > The kernel side burns extra bytes by copying it and extra branches to handle it.
> > > > May be drop it as well?
> > > I feel like the program benefits from it, there is no need to go back and
> > > re-parse that (and in the skb case, this data is already pulled). I was
> > > also thinking about re-purposing flow_keys->n_proto for that (instead
> > > of skb->protocol), so it functions as input and output, maybe that's a
> > > more clear way to do it.
> > 
> > Are you saying that skb-less and skb flow dissector progs are looking
> > at different positions into the packet ?
> No, sorry for confusion, they are both called to parse (optional) L2-vlan
> and L3+ headers. However, with-skb case can be called with l2-vlan
> parsed (post RFS) or with l2-vlan unparsed (RFS). The vlan is pulled in
> __netif_receive_skb_core, but we can still invoke flow dissector prior to
> that when doing RFS (get_rps_cpu).
> 
> That's why have this 'if skb->vlan_present' check in the bpf_flow.c program
> (and then also manually test for ETH_P_8021Q/ETH_P_8021AD).
> 
> Let me try to post the patches to bpf tree somewhere this week, we
> can discuss the API changes there.

let's figure out what we disable for bpf/stable first.
Sound like skb->protocol isn't helping.
prog has to read it from eth header anyway. then let's drop it from ctx?
skb->vlan_present also seems to be misleading.
For normal processing skb->vlan_present means that there is a vlan hdr
in the packet, but for flow dissector is some sort of weird hint
whether skb-based dissector was pre- or post- rfs.
I think we need make vlan semantics unambiguous first.