netdev - Re: [PATCH bpf-next 03/12] net: Add IFLA_XDP_EGRESS for XDP programs in the egress path

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Date:   Sat, 01 Feb 2020 17:24:39 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>, David Ahern <dsahern@...il.com>
Cc:     David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        prashantbhole.linux@...il.com, jasowang@...hat.com,
        davem@...emloft.net, jbrouer@...hat.com, mst@...hat.com,
        toshiaki.makita1@...il.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 bpf-next 03/12] net: Add IFLA_XDP_EGRESS for XDP programs in the egress path

[ skipping the first part to just comment on the below: ]

> I'm weary of partially implemented XDP features, EGRESS prog does us
> no good when most drivers didn't yet catch up with the REDIRECTs.

I kinda agree with this; but on the other hand, if we have to wait for
all drivers to catch up, that would mean we couldn't add *anything* new
that requires driver changes, which is not ideal either :/

> And we're adding this before we considered the queuing problem.
>
> But if I'm alone in thinking this, and I'm not convincing anyone we
> can move on :)

I do share your concern that this will end up being incompatible with
whatever solution we end up with for queueing. However, I don't
necessarily think it will: I view the XDP egress hook as something that
in any case will run *after* packets are dequeued from whichever
intermediate queueing it has been through (if any). I think such a hook
is missing in any case; for instance, it's currently impossible to
implement something like CoDel (which needs to know how long a packet
spent in the queue) in eBPF.

-Toke