[xdp-hints] Re: [RFC bpf-next v2 04/14] veth: Support rx timestamp metadata for xdp

[Stanislav Fomichev <sdf@google.com>]

On Wed, Nov 9, 2022 at 3:21 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Stanislav Fomichev <sdf@google.com> writes:
>
> > xskxceiver conveniently setups up veth pairs so it seems logical
> > to use veth as an example for some of the metadata handling.
> >
> > We timestamp skb right when we "receive" it, store its
> > pointer in new veth_xdp_buff wrapper and generate BPF bytecode to
> > reach it from the BPF program.
> >
> > This largely follows the idea of "store some queue context in
> > the xdp_buff/xdp_frame so the metadata can be reached out
> > from the BPF program".
> >
> > Cc: John Fastabend <john.fastabend@gmail.com>
> > Cc: David Ahern <dsahern@gmail.com>
> > Cc: Martin KaFai Lau <martin.lau@linux.dev>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Willem de Bruijn <willemb@google.com>
> > Cc: Jesper Dangaard Brouer <brouer@redhat.com>
> > Cc: Anatoly Burakov <anatoly.burakov@intel.com>
> > Cc: Alexander Lobakin <alexandr.lobakin@intel.com>
> > Cc: Magnus Karlsson <magnus.karlsson@gmail.com>
> > Cc: Maryam Tahhan <mtahhan@redhat.com>
> > Cc: xdp-hints@xdp-project.net
> > Cc: netdev@vger.kernel.org
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > ---
> >  drivers/net/veth.c | 31 +++++++++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> >
> > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > index 917ba57453c1..0e629ceb087b 100644
> > --- a/drivers/net/veth.c
> > +++ b/drivers/net/veth.c
> > @@ -25,6 +25,7 @@
> >  #include <linux/filter.h>
> >  #include <linux/ptr_ring.h>
> >  #include <linux/bpf_trace.h>
> > +#include <linux/bpf_patch.h>
> >  #include <linux/net_tstamp.h>
> >
> >  #define DRV_NAME     "veth"
> > @@ -118,6 +119,7 @@ static struct {
> >
> >  struct veth_xdp_buff {
> >       struct xdp_buff xdp;
> > +     struct sk_buff *skb;
> >  };
> >
> >  static int veth_get_link_ksettings(struct net_device *dev,
> > @@ -602,6 +604,7 @@ static struct xdp_frame *veth_xdp_rcv_one(struct veth_rq *rq,
> >
> >               xdp_convert_frame_to_buff(frame, xdp);
> >               xdp->rxq = &rq->xdp_rxq;
> > +             vxbuf.skb = NULL;
> >
> >               act = bpf_prog_run_xdp(xdp_prog, xdp);
> >
> > @@ -826,6 +829,7 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
> >
> >       orig_data = xdp->data;
> >       orig_data_end = xdp->data_end;
> > +     vxbuf.skb = skb;
> >
> >       act = bpf_prog_run_xdp(xdp_prog, xdp);
> >
> > @@ -942,6 +946,7 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
> >                       struct sk_buff *skb = ptr;
> >
> >                       stats->xdp_bytes += skb->len;
> > +                     __net_timestamp(skb);
> >                       skb = veth_xdp_rcv_skb(rq, skb, bq, stats);
> >                       if (skb) {
> >                               if (skb_shared(skb) || skb_unclone(skb, GFP_ATOMIC))
> > @@ -1665,6 +1670,31 @@ static int veth_xdp(struct net_device *dev, struct netdev_bpf *xdp)
> >       }
> >  }
> >
> > +static void veth_unroll_kfunc(const struct bpf_prog *prog, u32 func_id,
> > +                           struct bpf_patch *patch)
> > +{
> > +     if (func_id == xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_TIMESTAMP_SUPPORTED)) {
> > +             /* return true; */
> > +             bpf_patch_append(patch, BPF_MOV64_IMM(BPF_REG_0, 1));
> > +     } else if (func_id == xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_TIMESTAMP)) {
> > +             bpf_patch_append(patch,
> > +                     /* r5 = ((struct veth_xdp_buff *)r1)->skb; */
> > +                     BPF_LDX_MEM(BPF_DW, BPF_REG_5, BPF_REG_1,
> > +                                 offsetof(struct veth_xdp_buff, skb)),
> > +                     /* if (r5 == NULL) { */
> > +                     BPF_JMP_IMM(BPF_JNE, BPF_REG_5, 0, 2),
> > +                     /*      return 0; */
> > +                     BPF_MOV64_IMM(BPF_REG_0, 0),
> > +                     BPF_JMP_A(1),
> > +                     /* } else { */
> > +                     /*      return ((struct sk_buff *)r5)->tstamp; */
> > +                     BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_5,
> > +                                 offsetof(struct sk_buff, tstamp)),
> > +                     /* } */
>
> I don't think it's realistic to expect driver developers to write this
> level of BPF instructions for everything. With the 'patch' thing it
> should be feasible to write some helpers that driver developers can use,
> right? E.g., this one could be:
>
> bpf_read_context_member_u64(size_t ctx_offset, size_t member_offset)
>
> called as:
>
> bpf_read_context_member_u64(offsetof(struct veth_xdp_buff, skb), offsetof(struct sk_buff, tstamp));
>
> or with some macro trickery we could even hide the offsetof so you just
> pass in types and member names?

Definitely; let's start with the one you're proposing, we'll figure
out the rest as we go; thx!

> -Toke
>