netdev - Re: [PATCH bpf-next 1/5] libbpf: support XDP_SHARED

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Date:   Fri, 8 Nov 2019 19:19:18 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     William Tu <u9012063@...il.com>
Cc:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/5] libbpf: support XDP_SHARED_UMEM with
 external XDP program

On Fri, Nov 8, 2019 at 7:03 PM William Tu <u9012063@...il.com> wrote:
>
> Hi Magnus,
>
> Thanks for the patch.
>
> On Thu, Nov 07, 2019 at 06:47:36PM +0100, Magnus Karlsson wrote:
> > Add support in libbpf to create multiple sockets that share a single
> > umem. Note that an external XDP program need to be supplied that
> > routes the incoming traffic to the desired sockets. So you need to
> > supply the libbpf_flag XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD and load
> > your own XDP program.
> >
> > Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
> > ---
> >  tools/lib/bpf/xsk.c | 27 +++++++++++++++++----------
> >  1 file changed, 17 insertions(+), 10 deletions(-)
> >
> > diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> > index 86c1b61..8ebd810 100644
> > --- a/tools/lib/bpf/xsk.c
> > +++ b/tools/lib/bpf/xsk.c
> > @@ -586,15 +586,21 @@ int xsk_socket__create(struct xsk_socket **xsk_ptr, const char *ifname,
> >       if (!umem || !xsk_ptr || !rx || !tx)
> >               return -EFAULT;
> >
> > -     if (umem->refcount) {
> > -             pr_warn("Error: shared umems not supported by libbpf.\n");
> > -             return -EBUSY;
> > -     }
> > -
> >       xsk = calloc(1, sizeof(*xsk));
> >       if (!xsk)
> >               return -ENOMEM;
> >
> > +     err = xsk_set_xdp_socket_config(&xsk->config, usr_config);
> > +     if (err)
> > +             goto out_xsk_alloc;
> > +
> > +     if (umem->refcount &&
> > +         !(xsk->config.libbpf_flags & XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD)) {
> > +             pr_warn("Error: shared umems not supported by libbpf supplied XDP program.\n");
>
> Why can't we use the existing default one in libbpf?
> If users don't want to redistribute packet to different queue,
> then they can still use the libbpf default one.

Is there any point in creating two or more sockets tied to the same
umem and directing all traffic to just one socket? IMHO, I believe
that most users in this case would want to distribute the packets over
the sockets in some way. I also think that users might be unpleasantly
surprised if they create multiple sockets and all packets only get to
a single socket because libbpf loaded an XDP program that makes little
sense in the XDP_SHARED_UMEM case. If we force them to supply an XDP
program, they need to make this decision. I also wanted to extend the
sample with an explicit user loaded XDP program as an example of how
to do this. What do you think?

/Magnus

> William
> > +             err = -EBUSY;
> > +             goto out_xsk_alloc;
> > +     }
> > +
> >       if (umem->refcount++ > 0) {
> >               xsk->fd = socket(AF_XDP, SOCK_RAW, 0);
> >               if (xsk->fd < 0) {
> > @@ -616,10 +622,6 @@ int xsk_socket__create(struct xsk_socket **xsk_ptr, const char *ifname,
> >       memcpy(xsk->ifname, ifname, IFNAMSIZ - 1);
> >       xsk->ifname[IFNAMSIZ - 1] = '\0';
> >
> > -     err = xsk_set_xdp_socket_config(&xsk->config, usr_config);
> > -     if (err)
> > -             goto out_socket;
> > -
> >       if (rx) {
> >               err = setsockopt(xsk->fd, SOL_XDP, XDP_RX_RING,
> >                                &xsk->config.rx_size,
> > @@ -687,7 +689,12 @@ int xsk_socket__create(struct xsk_socket **xsk_ptr, const char *ifname,
> >       sxdp.sxdp_family = PF_XDP;
> >       sxdp.sxdp_ifindex = xsk->ifindex;
> >       sxdp.sxdp_queue_id = xsk->queue_id;
> > -     sxdp.sxdp_flags = xsk->config.bind_flags;
> > +     if (umem->refcount > 1) {
> > +             sxdp.sxdp_flags = XDP_SHARED_UMEM;
> > +             sxdp.sxdp_shared_umem_fd = umem->fd;
> > +     } else {
> > +             sxdp.sxdp_flags = xsk->config.bind_flags;
> > +     }
> >
> >       err = bind(xsk->fd, (struct sockaddr *)&sxdp, sizeof(sxdp));
> >       if (err) {
> > --
> > 2.7.4
> >