netdev - Re: [PATCH bpf-next 0/7] Add XDP_ATTACH bind() flag to AF

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Message-ID: <CAJ+HfNg6rbj7RvE67d4ryL1Yg1-qJSyTjnBJQY-_t93YbbZYmA@mail.gmail.com>
Date:   Fri, 7 Dec 2018 16:27:18 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        William Tu <u9012063@...il.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>
Subject: Re: [PATCH bpf-next 0/7] Add XDP_ATTACH bind() flag to AF_XDP sockets

Den fre 7 dec. 2018 kl 14:42 skrev Jesper Dangaard Brouer <brouer@...hat.com>:
>
> On Fri,  7 Dec 2018 12:44:24 +0100
> Björn Töpel <bjorn.topel@...il.com> wrote:
>
> > The rationale behind attach is performance and ease of use. Many XDP
> > socket users just need a simple way of creating/binding a socket and
> > receiving frames right away without loading an XDP program.
> >
> > XDP_ATTACH adds a mechanism we call "builtin XDP program" that simply
> > is a kernel provided XDP program that is installed to the netdev when
> > XDP_ATTACH is being passed as a bind() flag.
> >
> > The builtin program is the simplest program possible to redirect a
> > frame to an attached socket. In restricted C it would look like this:
> >
> >   SEC("xdp")
> >   int xdp_prog(struct xdp_md *ctx)
> >   {
> >         return bpf_xsk_redirect(ctx);
> >   }
> >
> > The builtin program loaded via XDP_ATTACH behaves, from an
> > install-to-netdev/uninstall-from-netdev point of view, differently
> > from regular XDP programs. The easiest way to look at it is as a
> > 2-level hierarchy, where regular XDP programs has precedence over the
> > builtin one.
> >
> > If no regular XDP program is installed to the netdev, the builtin will
> > be install. If the builtin program is installed, and a regular is
> > installed, regular XDP program will have precedence over the builtin
> > one.
> >
> > Further, if a regular program is installed, and later removed, the
> > builtin one will automatically be installed.
> >
> > The sxdp_flags field of struct sockaddr_xdp gets two new options
> > XDP_BUILTIN_SKB_MODE and XDP_BUILTIN_DRV_MODE, which maps to the
> > corresponding XDP netlink install flags.
> >
> > The builtin XDP program functionally adds even more complexity to the
> > already hard to read dev_change_xdp_fd. Maybe it would be simpler to
> > store the program in the struct net_device together with the install
> > flags instead of calling the ndo_bpf multiple times?
>
> (As far as I can see from reading the code, correct me if I'm wrong.)
>
> If an AF_XDP program uses XDP_ATTACH, then it installs the
> builtin-program as the XDP program on the "entire" device.  That means
> all RX-queues will call this XDP-bpf program (indirect call), and it is
> actually only relevant for the specific queue_index.  Yes, the helper
> call does check that the 'xdp->rxq->queue_index' for an attached 'xsk'
> and return XDP_PASS if it is NULL:
>

A side-note: What one can do, which I did for the plumbers work, is
bypassing the indirect call in bpf_prog_run_xdp by doing a "if the XDP
program is a builtin, call internal_bpf_xsk_redirect directly". Then,
the XDP_PASS path wont be taxed by the indirect call -- but only for
this special XDP_ATTACH program. And you'll still get an additional
if-statement in for the skb-path.


Björn

> +BPF_CALL_1(bpf_xdp_xsk_redirect, struct xdp_buff *, xdp)
> +{
> +       struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> +       struct xdp_sock *xsk;
> +
> +       xsk = READ_ONCE(xdp->rxq->dev->_rx[xdp->rxq->queue_index].xsk);
> +       if (xsk) {
> +               ri->xsk = xsk;
> +               return XDP_REDIRECT;
> +       }
> +
> +       return XDP_PASS;
> +}
>
> Why do every normal XDP_PASS packet have to pay this overhead (indirect
> call), when someone loads an AF_XDP socket program?  The AF_XDP socket
> is tied hard and only relevant to a specific RX-queue (which is why we
> get a performance boost due to SPSC queues).
>
> I acknowledge there is a need for this, but this use-case shows there
> is a need for attaching XDP programs per RX-queue basis.
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer