Modular match fields for tables.

[jonathan-dilorenzo]

For P4 LDWG, I promised I'd produce a toy example to illustrate this need, but I found a real example of ours from a year or so ago was open-sourced (though it looks much worse now).

Some possible thoughts on how I might like it to look:

• Literally make tables modifiable within their control block and allow something like 'AddAction' and 'AddKey':

  table acl_ingress_table {
    key = {
       *All the shared keys*
    }
    actions = {
       *All the shared actions*
    }
  }

  if ([some_metadata_saying_that_this_is_a_middleblock] == true) {
      acl_ingress_table.AddKey(
        headers.arp.target_proto_addr : ternary @name("arp_tpa") @id(16)
            @composite_field(
                @sai_udf(base=SAI_UDF_BASE_L3, offset=24, length=2),
                @sai_udf(base=SAI_UDF_BASE_L3, offset=26, length=2)
            ) @format(IPV4_ADDRESS);
        )
  }
  ...

• Make tables themselves parameterizable on variables with different types. I think we'd want something like bit<0> to effectively act as a non-match key in this world:

  table acl_ingress_table(my_icmp_key) {
    key = {
       *All the shared keys*
      my_icmp_type : ternary @name("icmp_type") @id(19)
          @sai_field(SAI_ACL_TABLE_ATTR_FIELD_ICMP_TYPE);
       ...
    }
  }
  bit<0> dummy_match_key;
  if ([some_metadata_saying_that_this_is_a_middleblock] == true) {
    acl_ingress_table.apply(dummy_match_key);
  } else {
     acl_ingress_table.apply(headers.icmp.type);
  }
  ...

• Make the control parameterizable on variables with different types and add the same bit<0> behavior as above. I'm actually not sure where this would fail today:

control acl_ingress(in my_icmp_key, ...) {
  table acl_ingress_table {
    key = {
       *All the shared keys*
      my_icmp_type : ternary @name("icmp_type") @id(19)
          @sai_field(SAI_ACL_TABLE_ATTR_FIELD_ICMP_TYPE);
       ...
    }
  }
}

control middleblock_ingress(...) {
  bit<0> dummy_match_key;
  apply {
     ...
    acl_ingress.apply(dummy_match_key, ...);
  }
} 
control fbr_ingress(...) {
  apply {
     ...
    acl_ingress.apply(headers.icmp.type, ...);
  }
}