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

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

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".

--Jesper