[xdp-hints] Re: [PATCH bpf-next 05/11] veth: Support rx timestamp metadata for xdp

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Nov 17, 2022 at 3:32 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Stanislav Fomichev <sdf@google.com> writes:
>
> >> > Doesn't look like the descriptors are as nice as you're trying to
> >> > paint them (with clear hash/csum fields) :-) So not sure how much
> >> > CO-RE would help.
> >> > At least looking at mlx4 rx_csum, the driver consults three different
> >> > sets of flags to figure out the hash_type. Or am I just unlucky with
> >> > mlx4?
> >>
> >> Which part are you talking about ?
> >>         hw_checksum = csum_unfold((__force __sum16)cqe->checksum);
> >> is trivial enough for bpf prog to do if it has access to 'cqe' pointer
> >> which is what John is proposing (I think).
> >
> > I'm talking about mlx4_en_process_rx_cq, the caller of that check_csum.
> > In particular: if (likely(dev->features & NETIF_F_RXCSUM)) branch
> > I'm assuming we want to have hash_type available to the progs?
>
> I agree we should expose the hash_type, but that doesn't actually look
> to be that complicated, see below.
>
> > But also, check_csum handles other corner cases:
> > - short_frame: we simply force all those small frames to skip checksum complete
> > - get_fixed_ipv6_csum: In IPv6 packets, hw_checksum lacks 6 bytes from
> > IPv6 header
> > - get_fixed_ipv4_csum: Although the stack expects checksum which
> > doesn't include the pseudo header, the HW adds it
> >
> > So it doesn't look like we can just unconditionally use cqe->checksum?
> > The driver does a lot of massaging around that field to make it
> > palatable.
>
> Poking around a bit in the other drivers, AFAICT it's only a subset of
> drivers that support CSUM_COMPLETE at all; for instance, the Intel
> drivers just set CHECKSUM_UNNECESSARY for TCP/UDP/SCTP. I think the
> CHECKSUM_UNNECESSARY is actually the most important bit we'd want to
> propagate?
>
> AFAICT, the drivers actually implementing CHECKSUM_COMPLETE need access
> to other data structures than the rx descriptor to determine the status
> of the checksum (mlx4 looks at priv->flags, mlx5 checks rq->state), so
> just exposing the rx descriptor to BPF as John is suggesting does not
> actually give the XDP program enough information to act on the checksum
> field on its own. We could still have a separate kfunc to just expose
> the hw checksum value (see below), but I think it probably needs to be
> paired with other kfuncs to be useful.
>
> Looking at the mlx4 code, I think the following mapping to kfuncs (in
> pseudo-C) would give the flexibility for XDP to access all the bits it
> needs, while inlining everything except getting the full checksum for
> non-TCP/UDP traffic. An (admittedly cursory) glance at some of the other
> drivers (mlx5, ice, i40e) indicates that this would work for those
> drivers as well.
>
>
> bpf_xdp_metadata_rx_hash_supported() {
>   return dev->features & NETIF_F_RXHASH;
> }
>
> bpf_xdp_metadata_rx_hash() {
>   return be32_to_cpu(cqe->immed_rss_invalid);
> }
>
> bpf_xdp_metdata_rx_hash_type() {
>   if (likely(dev->features & NETIF_F_RXCSUM) &&
>       (cqe->status & cpu_to_be16(MLX4_CQE_STATUS_TCP | MLX4_CQE_STATUS_UDP)) &&
>         (cqe->status & cpu_to_be16(MLX4_CQE_STATUS_IPOK)) &&
>           cqe->checksum == cpu_to_be16(0xffff))
>      return PKT_HASH_TYPE_L4;
>
>    return PKT_HASH_TYPE_L3;
> }
>
> bpf_xdp_metadata_rx_csum_supported() {
>   return dev->features & NETIF_F_RXCSUM;
> }
>
> bpf_xdp_metadata_rx_csum_level() {
>         if ((cqe->status & cpu_to_be16(MLX4_CQE_STATUS_TCP |
>                                        MLX4_CQE_STATUS_UDP)) &&
>             (cqe->status & cpu_to_be16(MLX4_CQE_STATUS_IPOK)) &&
>             cqe->checksum == cpu_to_be16(0xffff))
>             return CHECKSUM_UNNECESSARY;
>
>         if (!(priv->flags & MLX4_EN_FLAG_RX_CSUM_NON_TCP_UDP &&
>               (cqe->status & cpu_to_be16(MLX4_CQE_STATUS_IP_ANY))) &&
>               !short_frame(len))
>             return CHECKSUM_COMPLETE; /* we could also omit this case entirely */
>
>         return CHECKSUM_NONE;
> }
>
> /* this one could be called by the metadata_to_skb code */
> bpf_xdp_metadata_rx_csum_full() {
>   return check_csum() /* BPF_CALL this after refactoring so it is skb-agnostic */
> }
>
> /* this one would be for people like John who want to re-implement
>  * check_csum() themselves */
> bpf_xdp_metdata_rx_csum_raw() {
>   return cqe->checksum;
> }

Are you proposing a bunch of per-driver kfuncs that bpf prog will call.
If so that works, but bpf prog needs to pass dev and cqe pointers
into these kfuncs, so they need to be exposed to the prog somehow.
Probably through xdp_md ?
This way we can have both: bpf prog reading cqe fields directly
and using kfuncs to access things.
Inlining of kfuncs should be done generically.
It's not a driver job to convert native asm into bpf asm.