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

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Date:   Mon, 18 May 2020 11:08:59 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     David Ahern <dsahern@...il.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

David Ahern <dsahern@...il.com> writes:

> 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.

I can see your point that fixing up the whole skb after the program has
run is not a good idea. But to me that just indicates that the hook is
in the wrong place: that it really should be in the driver, executed at
a point where the skb data structure is no longer necessary (similar to
how the ingress hook is before the skb is generated).

Otherwise, what you're proposing is not an egress hook, but rather a
'post-REDIRECT hook', which is strictly less powerful. This may or may
not be useful in its own right, but let's not pretend it's a full egress
hook. Personally I feel that the egress hook is what we should be going
for, not this partial thing.

-Toke