[xdp-hints] Re: [PATCH bpf-next v3 03/12] bpf: XDP metadata RX kfuncs

[Stanislav Fomichev <sdf@google.com>]

On Wed, Dec 7, 2022 at 9:00 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> The offload tests still pass after this, right?

Yeah, had to bring them back in shape just for the purpose of making
sure they're still happy:
https://lore.kernel.org/bpf/20221206232739.2504890-1-sdf@google.com/

> TBH I don't remember this code well enough to spot major issues.

No worries! Appreciate the review and the comments on consistency; I'm
also mostly unaware how this whole offloading works :-)

> On Mon,  5 Dec 2022 18:45:45 -0800 Stanislav Fomichev wrote:
> > There is an ndo handler per kfunc, the verifier replaces a call to the
> > generic kfunc with a call to the per-device one.
> >
> > For XDP, we define a new kfunc set (xdp_metadata_kfunc_ids) which
> > implements all possible metatada kfuncs. Not all devices have to
> > implement them. If kfunc is not supported by the target device,
> > the default implementation is called instead.
> >
> > Upon loading, if BPF_F_XDP_HAS_METADATA is passed via prog_flags,
> > we treat prog_index as target device for kfunc resolution.
>
> > @@ -2476,10 +2477,18 @@ void bpf_offload_dev_netdev_unregister(struct bpf_offload_dev *offdev,
> >                                      struct net_device *netdev);
> >  bool bpf_offload_dev_match(struct bpf_prog *prog, struct net_device *netdev);
> >
> > +void *bpf_offload_resolve_kfunc(struct bpf_prog *prog, u32 func_id);
>
> There seems to be some mis-naming going on. I expected:
>
>   offloaded =~ nfp
>   dev_bound == XDP w/ funcs
>
> *_offload_resolve_kfunc looks misnamed? Unless you want to resolve
> for HW offload?

Yeah, I had the same expectations, but I was also assuming that this
bpf_offload_resolve_kfunc might also at some point handle offloaded
metadata kfuncs.
But looking at it again, agree that the following looks a bit off:

if (bpf_prog_is_dev_bound()) {
   xxx = bpf_offload_resolve_kfunc()
}

Let me use the dev_bound prefix more consistently here and in the
other places you've pointed out.

> >  void unpriv_ebpf_notify(int new_state);
> >
> >  #if defined(CONFIG_NET) && defined(CONFIG_BPF_SYSCALL)
> >  int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr);
> > +void bpf_offload_bound_netdev_unregister(struct net_device *dev);
>
> ditto: offload_bound is a mix of terms no?

Ack, will do bpf_dev_bound_netdev_unregister here, thanks!

> > @@ -1611,6 +1612,10 @@ struct net_device_ops {
> >       ktime_t                 (*ndo_get_tstamp)(struct net_device *dev,
> >                                                 const struct skb_shared_hwtstamps *hwtstamps,
> >                                                 bool cycles);
> > +     bool                    (*ndo_xdp_rx_timestamp_supported)(const struct xdp_md *ctx);
> > +     u64                     (*ndo_xdp_rx_timestamp)(const struct xdp_md *ctx);
> > +     bool                    (*ndo_xdp_rx_hash_supported)(const struct xdp_md *ctx);
> > +     u32                     (*ndo_xdp_rx_hash)(const struct xdp_md *ctx);
> >  };
>
> Is this on the fast path? Can we do an indirection?

No, we resolve them at load time from "generic"
bpf_xdp_metadata_rx_<xxx> to ndo_xdp_rx_<xxx>.

> Put these ops in their own struct and add a pointer to that struct
> in net_device_ops? Purely for grouping reasons because the netdev
> ops are getting orders of magnitude past the size where you can
> actually find stuff in this struct.

Oh, great idea, will do!

> >       bpf_free_used_maps(aux);
> >       bpf_free_used_btfs(aux);
> > -     if (bpf_prog_is_offloaded(aux))
> > +     if (bpf_prog_is_dev_bound(aux))
> >               bpf_prog_offload_destroy(aux->prog);
>
> This also looks a touch like a mix of terms (condition vs function
> called).

Here, not sure, open to suggestions. These
bpf_prog_offload_init/bpf_prog_offload_destroy are generic enough
(now) that I'm calling them for both dev_bound/offloaded.

The following paths trigger for both offloaded/dev_bound cases:

if (bpf_prog_is_dev_bound()) bpf_prog_offload_init();
if (bpf_prog_is_dev_bound()) bpf_prog_offload_destroy();

Do you think it's worth it having completely separate
dev_bound/offloaded paths? Or, alternatively, can rename to
bpf_prog_dev_bound_{init,destroy} but still handle both cases?

> > +static int __bpf_offload_init(void);
> > +static int __bpf_offload_dev_netdev_register(struct bpf_offload_dev *offdev,
> > +                                          struct net_device *netdev);
> > +static void __bpf_offload_dev_netdev_unregister(struct bpf_offload_dev *offdev,
> > +                                             struct net_device *netdev);
>
> fwd declarations are yuck

SG, will move them here instead.

> >  static int bpf_dev_offload_check(struct net_device *netdev)
> >  {
> >       if (!netdev)
> > @@ -87,13 +93,17 @@ int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr)
> >           attr->prog_type != BPF_PROG_TYPE_XDP)
> >               return -EINVAL;
> >
> > -     if (attr->prog_flags)
> > +     if (attr->prog_flags & ~BPF_F_XDP_HAS_METADATA)
> >               return -EINVAL;
> >
> >       offload = kzalloc(sizeof(*offload), GFP_USER);
> >       if (!offload)
> >               return -ENOMEM;
> >
> > +     err = __bpf_offload_init();
> > +     if (err)
> > +             return err;
>
> leaks offload

Oops, let me actually move this to late_initcall as you suggest below.

> > @@ -209,6 +233,19 @@ bpf_prog_offload_remove_insns(struct bpf_verifier_env *env, u32 off, u32 cnt)
> >       up_read(&bpf_devs_lock);
> >  }
> >
> > +static void maybe_remove_bound_netdev(struct net_device *dev)
> > +{
>
> func name prefix ?

Good point, will rename to bpf_dev_bound_try_remove_netdev.

> > -struct bpf_offload_dev *
> > -bpf_offload_dev_create(const struct bpf_prog_offload_ops *ops, void *priv)
> > +static int __bpf_offload_init(void)
> >  {
> > -     struct bpf_offload_dev *offdev;
> >       int err;
> >
> >       down_write(&bpf_devs_lock);
> > @@ -680,12 +740,25 @@ bpf_offload_dev_create(const struct bpf_prog_offload_ops *ops, void *priv)
> >               err = rhashtable_init(&offdevs, &offdevs_params);
> >               if (err) {
> >                       up_write(&bpf_devs_lock);
> > -                     return ERR_PTR(err);
> > +                     return err;
> >               }
> >               offdevs_inited = true;
> >       }
> >       up_write(&bpf_devs_lock);
> >
> > +     return 0;
> > +}
>
> Would late_initcall() or some such not work for this?

Agreed, let's move it to the initcall instead.

> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 5b221568dfd4..862e03fcffa6 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -9228,6 +9228,10 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
> >                       NL_SET_ERR_MSG(extack, "Using device-bound program without HW_MODE flag is not supported");
>
> extack should get updated here, I reckon, maybe in previous patch

Oh, thanks for spotting, will fix.

> >                       return -EINVAL;
> >               }
> > +             if (bpf_prog_is_dev_bound(new_prog->aux) && !bpf_offload_dev_match(new_prog, dev)) {
>
> bound_dev_match() ?

Right, so this is another case where it works for both cases. Maybe
rename to bpf_dev_bound_match and use for both offloaded/dev_bound? Or
do you prefer completely separate paths?

> > +                     NL_SET_ERR_MSG(extack, "Cannot attach to a different target device");
>
> different than.. ?

Borrowing from netdevsim, lmk if the following won't work here:

"Program bound to different device"

> > +                     return -EINVAL;
> > +             }
> >               if (new_prog->expected_attach_type == BPF_XDP_DEVMAP) {
> >                       NL_SET_ERR_MSG(extack, "BPF_XDP_DEVMAP programs can not be attached to a device");
> >                       return -EINVAL;