netdev - Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress path

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Date:   Wed, 13 May 2020 13:37:53 -0600
From:   David Ahern <dsahern@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org,
        prashantbhole.linux@...il.com, brouer@...hat.com,
        daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com, andriin@...com,
        David Ahern <dahern@...italocean.com>
Subject: Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress path

On 5/13/20 4:43 AM, Toke Høiland-Jørgensen wrote:
> I don't like this. I makes the egress hook asymmetrical with the ingress
> hook (ingress hook sees all traffic, egress only some of it). If the
> performance hit of disabling GSO is the concern, maybe it's better to
> wait until we figure out how to deal with that (presumably by
> multi-buffer XDP)?

XDP is for accelerated networking. Disabling a h/w offload feature to
use a s/w feature is just wrong. But it is more than just disabling GSO,
and multi-buffer support for XDP is still not going to solve the
problem. XDP is free form allowing any packet modifications - pushing
and popping headers - and, for example, that blows up all of the skb
markers for mac, network, transport and their inner versions. Walking
the skb after an XDP program has run to reset the markers does not make
sense. Combine this with the generic xdp overhead (e.g., handling skb
clone and linearize), and the whole thing just does not make sense.

We have to accept there a lot of use cases / code paths that simply can
not be converted to work with both skbs and xdp_frames. The qdisc code
is one example. This is another. Requiring a tc program for the skb path
is an acceptable trade off.