netdev - Re: CONFIG_NET_TC_SKB

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Date:   Sat, 21 Sep 2019 20:18:13 -0700
From:   Pravin Shelar <pshelar@....org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Vlad Buslov <vladbu@...lanox.com>,
        David Miller <davem@...emloft.net>,
        Paul Blakey <paulb@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...nulli.us>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Simon Horman <simon.horman@...ronome.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Or Gerlitz <gerlitz.or@...il.com>
Subject: Re: CONFIG_NET_TC_SKB_EXT

On Fri, Sep 20, 2019 at 5:06 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 9/21/19 12:56 AM, Jakub Kicinski wrote:
> [...]
> >> I thought idea of stuffing things into skb extensions are only justified if
> >> it's not enabled by default for everyone. :(
> >>
> >>     [0] https://lore.kernel.org/netdev/CAHC9VhSz1_KA1tCJtNjwK26BOkGhKGbPT7v1O82mWPduvWwd4A@mail.gmail.com/T/#u
> >
> > The skb ext allocation is only done with GOTO_CHAIN, which AFAIK only
> > has practical use for offload.  We could perhaps add another static
> > branch there or move the OvS static branch out of the OvS module so
> > there are no linking issues?
> >
> > I personally have little sympathy for this piece of code, it is perhaps
> > the purest form of a wobbly narrow-use construct pushed into TC for HW
> > offload.
> >
> > Any suggestions on the way forward? :(
>
> Presumably there are no clean solutions here, but on the top of my head for
> this use case, you'd need to /own/ the underlying datapath anyway, so couldn't
> you program the OVS key->recirc_id based on skb->mark (or alternatively via
> skb->tc_index) which was previously set by tc ingress?
>

If we are going down this path, tc_index should be used. skb->mark is
already used in OVS flow match, overloading it for this purpose would
be complicated.