[xdp-hints] Re: [PATCH bpf RFC 1/4] xdp: rss hash types representation

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

On 28/03/2023 23.58, Stanislav Fomichev wrote:
> On 03/28, 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 (xdp_rss_hash_type) that can both
>> be seen as a number that is ordered according by ISO layer, and can be bit
>> masked to separate IPv4 and IPv6 types for L4 protocols. Room is available
>> for extending later while keeping these properties. This maps and unifies
>> difference to hardware specific hashes.
> 
> Looks good overall. Any reason we're making this specific layout?

One important goal is to have a simple/fast way to determining L3 vs L4,
because a L4 hash can be used for flow handling (e.g. load-balancing).

We below layout you can:

  if (rss_type & XDP_RSS_TYPE_L4_MASK)
	bool hw_hash_do_LB = true;

Or using it as a number:

  if (rss_type > XDP_RSS_TYPE_L4)
	bool hw_hash_do_LB = true;

I'm very open to changes to my "specific" layout.  I am in doubt if
using it as a number is the right approach and worth the trouble.

> Why not simply the following?
> 
> enum {
>      XDP_RSS_TYPE_NONE = 0,
>      XDP_RSS_TYPE_IPV4 = BIT(0),
>      XDP_RSS_TYPE_IPV6 = BIT(1),
>      /* IPv6 with extension header. */
>      /* let's note ^^^ it in the UAPI? */
>      XDP_RSS_TYPE_IPV6_EX = BIT(2),
>      XDP_RSS_TYPE_UDP = BIT(3),
>      XDP_RSS_TYPE_TCP = BIT(4),
>      XDP_RSS_TYPE_SCTP = BIT(5),

We know these bits for UDP, TCP, SCTP (and IPSEC) are exclusive, they
cannot be set at the same time, e.g. as a packet cannot both be UDP and
TCP.  Thus, using these bits as a number make sense to me, and is more
compact.

This BIT() approach also have the issue of extending it later (forward
compatibility).  As mentioned a common task will be to check if
hash-type is a L4 type.  See mlx5 [patch 4/4] needed to extend with
IPSEC. Notice how my XDP_RSS_TYPE_L4_MASK covers all the bits that this
can be extended with new L4 types, such that existing progs will still
work checking for L4 check.  It can of-cause be solved in the same way
for this BIT() approach by reserving some bits upfront in a mask.

> }
> 
> And then using XDP_RSS_TYPE_IPV4|XDP_RSS_TYPE_UDP vs 
> XDP_RSS_TYPE_IPV6|XXX ?

Do notice, that I already does some level of or'ing ("|") in this
proposal.  The main difference is that I hide this from the driver, and
kind of pre-combine the valid combination (enum's) drivers can select
from. I do get the point, and I think I will come up with a combined
solution based on your input.


The RSS hashing types and combinations comes from M$ standards:
  [1] 
https://learn.microsoft.com/en-us/windows-hardware/drivers/network/rss-hashing-types#ipv4-hash-type-combinations


>> This proposal change the kfunc API bpf_xdp_metadata_rx_hash() to return
>> this RSS hash type on success.
> 
>> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
>> ---
>>   include/net/xdp.h |   51 
>> +++++++++++++++++++++++++++++++++++++++++++++++++++
>>   net/core/xdp.c    |    4 +++-
>>   2 files changed, 54 insertions(+), 1 deletion(-)
> 
>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>> index 5393b3ebe56e..63f462f5ea7f 100644
>> --- a/include/net/xdp.h
>> +++ b/include/net/xdp.h
>> @@ -8,6 +8,7 @@
> 
>>   #include <linux/skbuff.h> /* skb_shared_info */
>>   #include <uapi/linux/netdev.h>
>> +#include <linux/bitfield.h>
> 
>>   /**
>>    * DOC: XDP RX-queue information
>> @@ -396,6 +397,56 @@ XDP_METADATA_KFUNC_xxx
>>   MAX_XDP_METADATA_KFUNC,
>>   };
> 
>> +/* For partitioning of xdp_rss_hash_type */
>> +#define RSS_L3        GENMASK(2,0) /* 3-bits = values between 1-7 */
>> +#define L4_BIT        BIT(3)       /* 1-bit - L4 indication */
>> +#define RSS_L4_IPV4    GENMASK(6,4) /* 3-bits */
>> +#define RSS_L4_IPV6    GENMASK(9,7) /* 3-bits */
>> +#define RSS_L4        GENMASK(9,3) /* = 7-bits - covering L4 
>> IPV4+IPV6 */
>> +#define L4_IPV6_EX_BIT    BIT(9)       /* 1-bit - L4 IPv6 with 
>> Extension hdr */
>> +                     /* 11-bits in total */
>> +
>> +/* The XDP RSS hash type (xdp_rss_hash_type) can both be seen as a number that
>> + * is ordered according by ISO layer, and can be bit masked to separate IPv4 and
>> + * IPv6 types for L4 protocols. Room is available for extending later while
>> + * keeping above properties, as this need to cover NIC hardware RSS types.
>> + */
>> +enum xdp_rss_hash_type {
>> +    XDP_RSS_TYPE_NONE            = 0,
>> +    XDP_RSS_TYPE_L2              = XDP_RSS_TYPE_NONE,
>> +
>> +    XDP_RSS_TYPE_L3_MASK         = RSS_L3,
>> +    XDP_RSS_TYPE_L3_IPV4         = FIELD_PREP_CONST(RSS_L3, 1),
>> +    XDP_RSS_TYPE_L3_IPV6         = FIELD_PREP_CONST(RSS_L3, 2),
>> +    XDP_RSS_TYPE_L3_IPV6_EX      = FIELD_PREP_CONST(RSS_L3, 4),
>> +
>> +    XDP_RSS_TYPE_L4_MASK         = RSS_L4,
>> +    XDP_RSS_TYPE_L4_SHIFT        = __bf_shf(RSS_L4),
>> +    XDP_RSS_TYPE_L4_MASK_EX      = RSS_L4 | L4_IPV6_EX_BIT,
>> +
>> +    XDP_RSS_TYPE_L4_IPV4_MASK    = RSS_L4_IPV4,
>> +    XDP_RSS_TYPE_L4_BIT          = L4_BIT,
>> +    XDP_RSS_TYPE_L4_IPV4_TCP     = L4_BIT|FIELD_PREP_CONST(RSS_L4_IPV4, 1),
>> +    XDP_RSS_TYPE_L4_IPV4_UDP     = L4_BIT|FIELD_PREP_CONST(RSS_L4_IPV4, 2),
>> +    XDP_RSS_TYPE_L4_IPV4_SCTP    = L4_BIT|FIELD_PREP_CONST(RSS_L4_IPV4, 3),
>> +
>> +    XDP_RSS_TYPE_L4_IPV6_MASK    = RSS_L4_IPV6,
>> +    XDP_RSS_TYPE_L4_IPV6_TCP     = L4_BIT|FIELD_PREP_CONST(RSS_L4_IPV6, 1),
>> +    XDP_RSS_TYPE_L4_IPV6_UDP     = L4_BIT|FIELD_PREP_CONST(RSS_L4_IPV6, 2),
>> +    XDP_RSS_TYPE_L4_IPV6_SCTP    = L4_BIT|FIELD_PREP_CONST(RSS_L4_IPV6, 3),
>> +
>> +    XDP_RSS_TYPE_L4_IPV6_EX_MASK = L4_IPV6_EX_BIT,
>> +    XDP_RSS_TYPE_L4_IPV6_TCP_EX  = XDP_RSS_TYPE_L4_IPV6_TCP|L4_IPV6_EX_BIT,
>> +    XDP_RSS_TYPE_L4_IPV6_UDP_EX  = XDP_RSS_TYPE_L4_IPV6_UDP|L4_IPV6_EX_BIT,
>> +    XDP_RSS_TYPE_L4_IPV6_SCTP_EX = XDP_RSS_TYPE_L4_IPV6_SCTP|L4_IPV6_EX_BIT,
>> +};
>> +#undef RSS_L3
>> +#undef L4_BIT
>> +#undef RSS_L4_IPV4
>> +#undef RSS_L4_IPV6
>> +#undef RSS_L4
>> +#undef L4_IPV6_EX_BIT
>> +
>>   #ifdef CONFIG_NET
>>   u32 bpf_xdp_metadata_kfunc_id(int id);
>>   bool bpf_dev_bound_kfunc_id(u32 btf_id);
>> diff --git a/net/core/xdp.c b/net/core/xdp.c
>> index 7133017bcd74..81d41df30695 100644
>> --- a/net/core/xdp.c
>> +++ b/net/core/xdp.c
>> @@ -721,12 +721,14 @@ __bpf_kfunc int 
>> bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *tim
>>    * @hash: Return value pointer.
>>    *
>>    * Return:
>> - * * Returns 0 on success or ``-errno`` on error.
>> + * * Returns (positive) RSS hash **type** on success or ``-errno`` on 
>> error.
>> + * * ``enum xdp_rss_hash_type`` : RSS hash type
>>    * * ``-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)
>>   {
>> +    BTF_TYPE_EMIT(enum xdp_rss_hash_type);
>>       return -EOPNOTSUPP;
>>   }
> 
> 
>