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

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Date:   Sat, 23 Mar 2019 09:05:31 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
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 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.