@antoninbas I have a question about direct_meter writes. I am trying to use the p4runtime API to craft an update message, but I am unable to get this particular write to work. The message I am seeing is
* At index 0: INVALID_ARGUMENT, 'Missing non-ternary field in match key'.

I am not exactly sure how to craft a match key here that makes sense. I tried various things but keep getting the same message. Have not found a similar example, other operations on direct externs seem to be reads. Any advice?

You are only setting the table_id field in meter_write.table_entry. This is not enough, every table entry has its own meter entry. You need to provide the full match key, like you would if you were doing a MODIFY on a TableEntry.

You need to be able to call set_match_key here, for the direct case: the index is replaced by a match key.

I see, so the direct_meter match key needs to match exactly with an existing, already installed match key?

For a write operation, yes.
The direct meter entry can also be configured at the time of inserting the table entry, to avoid 2 RPCs: https://github.com/p4lang/p4runtime/blob/90553b90a12ead5c19700e7fef21164dea5b6d22/proto/p4/v1/p4runtime.proto#L191

Thanks! I got something to work.
There is one edge case that is tricky. I am guessing the direct meter always requires an entry in the table to be configured? For example, if you have this construct:

table meter_table {
    actions = {
        meter_assign;
    }
    key = {
        h.eth_hdr.dst_addr: exact;
    }
    size = 16384;
    default_action = meter_assign();
    meters = table_attached_meter;
}

where meter_assign is executed as default action, one might have no entry on the table. I am guessing this case is not supported? I tried abusing the API with is_default_action but it did not work.

P4Runtime & PI support direct resources for the default table entry, but bmv2 does not.
When attempting this, an error should be returned and the simple_switch_grpc logs should read: [PI] bmv2 does not support direct meters for default entries.

Thanks, that was quite helpful.

Is evaluatedConditions unused?

Same here.

Is this still needed?