Re: [PATCH bpf RFC-V3 1/5] xdp: rss hash types representation

[Jesper Dangaard Brouer]

On 30/03/2023 20.35, Stanislav Fomichev wrote:
> On 03/30, Jesper Dangaard Brouer wrote:
> > The RSS hash type specifies what portion of packet data NIC hardware used
> > when calculating RSS hash value. The RSS types are focused on Internet
> > traffic protocols at OSI layers L3 and L4. L2 (e.g. ARP) often get hash
> > value zero and no RSS type. For L3 focused on IPv4 vs. IPv6, and L4
> > primarily TCP vs UDP, but some hardware supports SCTP.
>
> > Hardware RSS types are differently encoded for each hardware NIC. Most
> > hardware represent RSS hash type as a number. Determining L3 vs L4 often
> > requires a mapping table as there often isn't a pattern or sorting
> > according to ISO layer.
>
> > The patch introduce a XDP RSS hash type (enum xdp_rss_hash_type) that
> > contain combinations to be used by drivers, which gets build up with bits
> > from enum xdp_rss_type_bits. Both enum xdp_rss_type_bits and
> > xdp_rss_hash_type get exposed to BPF via BTF, and it is up to the
> > BPF-programmer to match using these defines.
>
> > This proposal change the kfunc API bpf_xdp_metadata_rx_hash() adding
> > a pointer value argument for provide the RSS hash type.
>
> > Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> > ---
> >   include/linux/netdevice.h |    3 ++-
> >   include/net/xdp.h         |   46 +++++++++++++++++++++++++++++++++++++++++++++
> >   net/core/xdp.c            |   10 +++++++++-
> >   3 files changed, 57 insertions(+), 2 deletions(-)

[...]
> > diff --git a/net/core/xdp.c b/net/core/xdp.c
> > index 528d4b37983d..38d2dee16b47 100644
> > --- a/net/core/xdp.c
> > +++ b/net/core/xdp.c
> > @@ -734,14 +734,22 @@ __bpf_kfunc int 
> > bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *tim
> >    * bpf_xdp_metadata_rx_hash - Read XDP frame RX hash.
> >    * @ctx: XDP context pointer.
> >    * @hash: Return value pointer.
> > + * @rss_type: Return value pointer for RSS type.
> > + *
> > + * The RSS hash type (@rss_type) specifies what portion of packet headers NIC
> > + * hardware were used when calculating RSS hash value.  The type combinations
> > + * are defined via &enum xdp_rss_hash_type and individual bits can be decoded
> > + * via &enum xdp_rss_type_bits.
> >    *
> >    * Return:
> >    * * Returns 0 on success or ``-errno`` on error.
> >    * * ``-EOPNOTSUPP`` : means device driver doesn't implement kfunc
> >    * * ``-ENODATA``    : means no RX-hash available for this frame
> >    */
> > -__bpf_kfunc int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, 
> > u32 *hash)
> > +__bpf_kfunc int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, 
> > u32 *hash,
> > +                     enum xdp_rss_hash_type *rss_type)
> >   {
>
> [..]
>
> > +    BTF_TYPE_EMIT(enum xdp_rss_type_bits);
>
> nit: Do we still need this with an extra argument?

Yes, unfortunately (compiler optimizes out enum xdp_rss_type_bits).
Do notice the difference xdp_rss_type_bits vs xdp_rss_hash_type.
We don't need it for "xdp_rss_hash_type" but need it for 
"xdp_rss_type_bits".

--Jesper