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

[Stanislav Fomichev <sdf@google.com>]

On Thu, Mar 30, 2023 at 11:56 AM Jesper Dangaard Brouer
<jbrouer@redhat.com> wrote:
>
>
> 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@redhat.com>
> >> ---
> >>   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".

Ah, I missed that. Then why not expose xdp_rss_type_bits?
Keep xdp_rss_hash_type for internal drivers' tables, and export the
enum with the bits?

> --Jesper
>