netdev - Re: [PATCH bpf-next v2 4/8] xsk: register XDP sockets at bind(), and add new AF

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Message-ID: <CAADnVQ+R5JHhqUFnB_o3nJkkkcEtvO_Vk+xSDFiqP9dZ9H6vxw@mail.gmail.com>
Date:   Wed, 20 Jan 2021 13:15:48 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>, maximmi@...dia.com,
        "David S. Miller" <davem@...emloft.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Ciara Loftus <ciara.loftus@...el.com>,
        weqaar.a.janjua@...el.com
Subject: Re: [PATCH bpf-next v2 4/8] xsk: register XDP sockets at bind(), and
 add new AF_XDP BPF helper

On Wed, Jan 20, 2021 at 12:26 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> This argument, however, I buy: bpf_redirect() is the single-purpose
> helper for redirecting to an ifindex, bpf_redirect_xsk() is the
> single-purpose helper for redirecting to an XSK, and bpf_redirect_map()
> is the generic one that does both of those and more. Fair enough,
> consider me convinced :)
>
> > A lot of back-and-forth for *one* if-statement, but it's kind of a
> > design thing for me. ;-)
>
> Surely you don't mean to imply that you have *better* things to do with
> your time than have a 10-emails-long argument over a single if
> statement? ;)

After reading this thread I think I have to pour cold water on the design.

The performance blip comes from hard coded assumptions:
+ queue_id = xdp->rxq->queue_index;
+ xs = READ_ONCE(dev->_rx[queue_id].xsk);

bpf can have specialized helpers, but imo this is beyond what's reasonable.
Please move such things into the program and try to make
bpf_redirect_map faster.

Making af_xdp non-root is orthogonal. If there is actual need for that
it has to be designed thoroughly and not presented as "this helper may
help to do that".
I don't think "may" will materialize unless people actually work
toward the goal of non-root.