[xdp-hints] Re: [PATCH bpf-next v2 2/8] bpf: XDP metadata RX kfuncs

[Stanislav Fomichev <sdf@google.com>]

On Mon, Nov 28, 2022 at 10:53 AM Stanislav Fomichev <sdf@google.com> wrote:
>
>  s
>
> On Fri, Nov 25, 2022 at 9:53 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >
> > Stanislav Fomichev <sdf@google.com> writes:
> >
> > > 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.
> >
> > BTW, this "the default implementation is called instead" bit is not
> > included in this version... :)
>
> fixup_xdp_kfunc_call should return 0 when the device doesn't have a
> kfunc defined and should fallback to the default kfunc implementation,
> right?
> Or am I missing something?
>
> > [...]
> >
> > > +#ifdef CONFIG_DEBUG_INFO_BTF
> > > +BTF_SET8_START(xdp_metadata_kfunc_ids)
> > > +#define XDP_METADATA_KFUNC(name, str) BTF_ID_FLAGS(func, str, 0)
> > > +XDP_METADATA_KFUNC_xxx
> > > +#undef XDP_METADATA_KFUNC
> > > +BTF_SET8_END(xdp_metadata_kfunc_ids)
> > > +
> > > +static const struct btf_kfunc_id_set xdp_metadata_kfunc_set = {
> > > +     .owner = THIS_MODULE,
> > > +     .set   = &xdp_metadata_kfunc_ids,
> > > +};
> > > +
> > > +u32 xdp_metadata_kfunc_id(int id)
> > > +{
> > > +     return xdp_metadata_kfunc_ids.pairs[id].id;
> > > +}
> > > +EXPORT_SYMBOL_GPL(xdp_metadata_kfunc_id);
> >
> > So I was getting some really weird values when testing (always getting a
> > timestamp value of '1'), and it turns out to be because this way of
> > looking up the ID doesn't work: The set is always sorted by the BTF ID,
> > not the order it was defined. Which meant that the mapping code got the
> > functions mixed up, and would call a different one instead (so the
> > timestamp value I was getting was really the return value of
> > rx_hash_enabled()).
> >
> > I fixed it by building a secondary lookup table as below; feel free to
> > incorporate that (or if you can come up with a better way, go ahead!).
>
> Interesting, will take a closer look. I took this pattern from
> BTF_SOCK_TYPE_xxx, which means that 'sorting by btf-id' is something
> BTF_SET8_START specific...
> But if it's sorted, probably easier to do a bsearch over this table
> than to build another one?

Ah, I see, there is no place to store an index :-( Maybe the following
is easier still?

diff --git a/net/core/xdp.c b/net/core/xdp.c
index e43f7d4ef4cf..8240805bfdb7 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -743,9 +743,15 @@ static const struct btf_kfunc_id_set
xdp_metadata_kfunc_set = {
        .set   = &xdp_metadata_kfunc_ids,
 };

+BTF_ID_LIST(xdp_metadata_kfunc_ids_unsorted)
+#define XDP_METADATA_KFUNC(name, str) BTF_ID(func, str)
+XDP_METADATA_KFUNC_xxx
+#undef XDP_METADATA_KFUNC
+
 u32 xdp_metadata_kfunc_id(int id)
 {
-       return xdp_metadata_kfunc_ids.pairs[id].id;
+       /* xdp_metadata_kfunc_ids is sorted and can't be used */
+       return xdp_metadata_kfunc_ids_unsorted[id];
 }
 EXPORT_SYMBOL_GPL(xdp_metadata_kfunc_id);



> > -Toke
> >
> > diff --git a/net/core/xdp.c b/net/core/xdp.c
> > index e43f7d4ef4cf..dc0a9644dacc 100644
> > --- a/net/core/xdp.c
> > +++ b/net/core/xdp.c
> > @@ -738,6 +738,15 @@ XDP_METADATA_KFUNC_xxx
> >  #undef XDP_METADATA_KFUNC
> >  BTF_SET8_END(xdp_metadata_kfunc_ids)
> >
> > +static struct xdp_metadata_kfunc_map {
> > +       const char *fname;
> > +       u32 btf_id;
> > +} xdp_metadata_kfunc_lookup_map[MAX_XDP_METADATA_KFUNC] = {
> > +#define XDP_METADATA_KFUNC(name, str) { .fname = __stringify(str) },
> > +XDP_METADATA_KFUNC_xxx
> > +#undef XDP_METADATA_KFUNC
> > +};
> > +
> >  static const struct btf_kfunc_id_set xdp_metadata_kfunc_set = {
> >         .owner = THIS_MODULE,
> >         .set   = &xdp_metadata_kfunc_ids,
> > @@ -745,13 +754,41 @@ static const struct btf_kfunc_id_set xdp_metadata_kfunc_set = {
> >
> >  u32 xdp_metadata_kfunc_id(int id)
> >  {
> > -       return xdp_metadata_kfunc_ids.pairs[id].id;
> > +       return xdp_metadata_kfunc_lookup_map[id].btf_id;
> >  }
> >  EXPORT_SYMBOL_GPL(xdp_metadata_kfunc_id);
> >
> >  static int __init xdp_metadata_init(void)
> >  {
> > -       return register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &xdp_metadata_kfunc_set);
> > +       const struct btf *btf;
> > +       int i, j, ret;
> > +
> > +       ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &xdp_metadata_kfunc_set);
> > +       if (ret)
> > +               return ret;
> > +
> > +       btf = bpf_get_btf_vmlinux();
> > +
> > +       for (i = 0; i < MAX_XDP_METADATA_KFUNC; i++) {
> > +               u32 btf_id = xdp_metadata_kfunc_ids.pairs[i].id;
> > +               const struct btf_type *t;
> > +               const char *name;
> > +
> > +               t = btf_type_by_id(btf, btf_id);
> > +               if (WARN_ON_ONCE(!t || !t->name_off))
> > +                       continue;
> > +
> > +               name = btf_name_by_offset(btf, t->name_off);
> > +
> > +               for (j = 0; j < MAX_XDP_METADATA_KFUNC; j++) {
> > +                       if (!strcmp(name, xdp_metadata_kfunc_lookup_map[j].fname)) {
> > +                               xdp_metadata_kfunc_lookup_map[j].btf_id = btf_id;
> > +                               break;
> > +                       }
> > +               }
> > +       }
> > +
> > +       return 0;
> >  }
> >  late_initcall(xdp_metadata_init);
> >  #else /* CONFIG_DEBUG_INFO_BTF */
> >