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

[Stanislav Fomichev <sdf@google.com>]

On Wed, Nov 16, 2022 at 11:03 AM John Fastabend
<john.fastabend@gmail.com> wrote:
>
> Toke Høiland-Jørgensen wrote:
> > Martin KaFai Lau <martin.lau@linux.dev> writes:
> >
> > > On 11/15/22 10:38 PM, John Fastabend wrote:
> > >>>>>>> +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)) {
> > >>>>>>> +             /* return ktime_get_mono_fast_ns(); */
> > >>>>>>> +             bpf_patch_append(patch, BPF_EMIT_CALL(ktime_get_mono_fast_ns));
> > >>>>>>> +     }
> > >>>>>>> +}
> > >>>>>>
> > >>>>>> So these look reasonable enough, but would be good to see some examples
> > >>>>>> of kfunc implementations that don't just BPF_CALL to a kernel function
> > >>>>>> (with those helper wrappers we were discussing before).
> > >>>>>
> > >>>>> Let's maybe add them if/when needed as we add more metadata support?
> > >>>>> xdp_metadata_export_to_skb has an example, and rfc 1/2 have more
> > >>>>> examples, so it shouldn't be a problem to resurrect them back at some
> > >>>>> point?
> > >>>>
> > >>>> Well, the reason I asked for them is that I think having to maintain the
> > >>>> BPF code generation in the drivers is probably the biggest drawback of
> > >>>> the kfunc approach, so it would be good to be relatively sure that we
> > >>>> can manage that complexity (via helpers) before we commit to this :)
> > >>>
> > >>> Right, and I've added a bunch of examples in v2 rfc so we can judge
> > >>> whether that complexity is manageable or not :-)
> > >>> Do you want me to add those wrappers you've back without any real users?
> > >>> Because I had to remove my veth tstamp accessors due to John/Jesper
> > >>> objections; I can maybe bring some of this back gated by some
> > >>> static_branch to avoid the fastpath cost?
> > >>
> > >> I missed the context a bit what did you mean "would be good to see some
> > >> examples of kfunc implementations that don't just BPF_CALL to a kernel
> > >> function"? In this case do you mean BPF code directly without the call?
> > >>
> > >> Early on I thought we should just expose the rx_descriptor which would
> > >> be roughly the same right? (difference being code embedded in driver vs
> > >> a lib) Trouble I ran into is driver code using seqlock_t and mutexs
> > >> which wasn't as straight forward as the simpler just read it from
> > >> the descriptor. For example in mlx getting the ts would be easy from
> > >> BPF with the mlx4_cqe struct exposed
> > >>
> > >> u64 mlx4_en_get_cqe_ts(struct mlx4_cqe *cqe)
> > >> {
> > >>          u64 hi, lo;
> > >>          struct mlx4_ts_cqe *ts_cqe = (struct mlx4_ts_cqe *)cqe;
> > >>
> > >>          lo = (u64)be16_to_cpu(ts_cqe->timestamp_lo);
> > >>          hi = ((u64)be32_to_cpu(ts_cqe->timestamp_hi) + !lo) << 16;
> > >>
> > >>          return hi | lo;
> > >> }
> > >>
> > >> but converting that to nsec is a bit annoying,
> > >>
> > >> void mlx4_en_fill_hwtstamps(struct mlx4_en_dev *mdev,
> > >>                              struct skb_shared_hwtstamps *hwts,
> > >>                              u64 timestamp)
> > >> {
> > >>          unsigned int seq;
> > >>          u64 nsec;
> > >>
> > >>          do {
> > >>                  seq = read_seqbegin(&mdev->clock_lock);
> > >>                  nsec = timecounter_cyc2time(&mdev->clock, timestamp);
> > >>          } while (read_seqretry(&mdev->clock_lock, seq));
> > >>
> > >>          memset(hwts, 0, sizeof(struct skb_shared_hwtstamps));
> > >>          hwts->hwtstamp = ns_to_ktime(nsec);
> > >> }
> > >>
> > >> I think the nsec is what you really want.
> > >>
> > >> With all the drivers doing slightly different ops we would have
> > >> to create read_seqbegin, read_seqretry, mutex_lock, ... to get
> > >> at least the mlx and ice drivers it looks like we would need some
> > >> more BPF primitives/helpers. Looks like some more work is needed
> > >> on ice driver though to get rx tstamps on all packets.
> > >>
> > >> Anyways this convinced me real devices will probably use BPF_CALL
> > >> and not BPF insns directly.
> > >
> > > Some of the mlx5 path looks like this:
> > >
> > > #define REAL_TIME_TO_NS(hi, low) (((u64)hi) * NSEC_PER_SEC + ((u64)low))
> > >
> > > static inline ktime_t mlx5_real_time_cyc2time(struct mlx5_clock *clock,
> > >                                                u64 timestamp)
> > > {
> > >          u64 time = REAL_TIME_TO_NS(timestamp >> 32, timestamp & 0xFFFFFFFF);
> > >
> > >          return ns_to_ktime(time);
> > > }
> > >
> > > If some hints are harder to get, then just doing a kfunc call is better.
> >
> > Sure, but if we end up having a full function call for every field in
> > the metadata, that will end up having a significant performance impact
> > on the XDP data path (thinking mostly about the skb metadata case here,
> > which will collect several bits of metadata).
> >
> > > csum may have a better chance to inline?
> >
> > Yup, I agree. Including that also makes it possible to benchmark this
> > series against Jesper's; which I think we should definitely be doing
> > before merging this.
>
> Good point I got sort of singularly focused on timestamp because I have
> a use case for it now.
>
> Also hash is often sitting in the rx descriptor.

Ack, let me try to add something else (that's more inline-able) on the
rx side for a v2.

> >
> > > Regardless, BPF in-lining is a well solved problem and used in many
> > > bpf helpers already, so there are many examples in the kernel. I don't
> > > think it is necessary to block this series because of missing some
> > > helper wrappers for inlining. The driver can always start with the
> > > simpler kfunc call first and optimize later if some hints from the
> > > drivers allow it.
> >
> > Well, "solved" in the sense of "there are a few handfuls of core BPF
> > people who know how to do it". My concern is that we'll end up with
> > either the BPF devs having to maintain all these bits of BPF byte code
> > in all the drivers; or drivers just punting to regular function calls
> > because the inlining is too complicated, with sub-par performance as per
> > the above. I don't think we should just hand-wave this away as "solved",
> > but rather treat this as an integral part of the initial series.
>
> This was my motivation for pushing the rx_descriptor into the xdp_buff.
> At this point if I'm going to have a kfunc call into the driver and
> have the driver rewrite the code into some BPF instructions I would
> just assume maintain this as a library code where I can hook it
> into my BPF program directly from user space. Maybe a few drivers
> will support all the things I want to read, but we run on lots of
> hardware (mlx, intel, eks, azure, gke, etc) and its been a lot of work
> to just get the basic feature parity. I also don't want to run around
> writing driver code for each vendor if I can avoid it. Having raw
> access to the rx descriptor gets me the escape hatch where I can
> just do it myself. And the last piece of info from my point of view
> (Tetragon, Cilium) I can run whatever libs I want and freely upgrade
> libbpf and cilium/ebpf but have a lot less ability to get users
> to upgrade kernels outside the LTS they picked. Meaning I can
> add new things much easier if its lifted into BPF code placed
> by user space.
>
> I appreciate that it means I import the problem of hardware detection
> and BTF CO-RE on networking codes, but we've already solved these
> problems for other reasons. For example just configuring the timestamp
> is a bit of an exercise in does my hardware support timestamping
> and does it support timestamping the packets I care about,  e.g.
> all pkts, just ptp pkts, etc.
>
> I don't think they are mutual exclusive with this series though
> because I can't see how to write these timestamping logic directly
> in BPF. But for rxhash and csum it seems doable. My preference
> is to have both the kfuncs and expose the descriptor directly.
>
> .John