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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Stanislav Fomichev <sdf@google.com> writes:

> On Thu, Nov 17, 2022 at 8:59 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
>>
>> 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 ?

No, I didn't mean we should call per-driver kfuncs; the examples above
were meant to be examples of what the mlx4 driver would unrolls those
kfuncs to. Sorry that that wasn't clear.

> So far I'm doing:
>
> struct mlx4_xdp_buff {
>   struct xdp_buff xdp;
>   struct mlx4_cqe *cqe;
>   struct mlx4_en_dev *mdev;
> }
>
> And then the kfuncs get ctx (aka xdp_buff) as a sole argument and can
> find cqe/mdev via container_of.
>
> If we really need these to be exposed to the program, can we use
> Yonghong's approach from [0]?

I don't think we should expose them to the BPF program; I like your
approach of stuffing them next to the CTX pointer and de-referencing
that. This makes it up to the driver which extra objects it needs, and
the caller doesn't have to know/care.

I'm not vehemently opposed to *also* having the rx-desc pointer directly
accessible (in which case Yonghong's kfunc approach is probably fine).
However, as mentioned in my previous email, I doubt how useful that
descriptor itself will be...

>> 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.
>
> Ack. I can replace the unrolling with something that just resolves
> "generic" kfuncs to the per-driver implementation maybe? That would at
> least avoid netdev->ndo_kfunc_xxx indirect calls at runtime..

As stated above, I think we should keep the unrolling. If we end up with
an actual CALL instruction for every piece of metadata that's going to
suck performance-wise; unrolling is how we keep this fast enough! :)

-Toke