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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Tue, 3 Mar 2020 16:10:14 -0500
Message-ID: <CA+FuTSeL_psqzpB6hxSh6f1HnO_SrpED=71Y3HcyDweG2Y3sdg@mail.gmail.com>
Subject: Re: [PATCH v4] netdev attribute to control xdpgeneric skb linearization
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     Daniel Borkmann <daniel@iogearbox.net>,
        Luigi Rizzo <lrizzo@google.com>,
        Network Development <netdev@vger.kernel.org>,
        =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>,
        David Miller <davem@davemloft.net>, hawk@kernel.org,
        "Jubran, Samih" <sameehj@amazon.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>, bpf <bpf@vger.kernel.org>
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On Tue, Mar 3, 2020 at 3:50 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 3 Mar 2020 20:46:55 +0100 Daniel Borkmann wrote:
> > Thus, when the data/data_end test fails in generic XDP, the user can
> > call e.g. bpf_xdp_pull_data(xdp, 64) to make sure we pull in as much as
> > is needed w/o full linearization and once done the data/data_end can be
> > repeated to proceed. Native XDP will leave xdp->rxq->skb as NULL, but
> > later we could perhaps reuse the same bpf_xdp_pull_data() helper for
> > native with skb-less backing. Thoughts?

Something akin to pskb_may_pull sounds like a great solution to me.

Another approach would be a new xdp_action XDP_NEED_LINEARIZED that
causes the program to be restarted after linearization. But that is both
more expensive and less elegant.

Instead of a sysctl or device option, is this an optimization that
could be taken based on the program? Specifically, would XDP_FLAGS be
a path to pass a SUPPORT_SG flag along with the program? I'm not
entirely familiar with the XDP setup code, so this may be a totally
off. But from a quick read it seems like generic_xdp_install could
transfer such a flag to struct net_device.

> I'm curious why we consider a xdpgeneric-only addition. Is attaching
> a cls_bpf program noticeably slower than xdpgeneric?

This just should not be xdp*generic* only, but allow us to use any XDP
with large MTU sizes and without having to disable GRO. I'd still like a
way to be able to drop or modify packets before GRO, or to signal that
a type of packet should skip GRO.