netdev-genl: Elide napi_id when not present

There are at least two cases where napi_id may not present and the
napi_id should be elided:

1. Queues could be created, but napi_enable may not have been called
   yet. In this case, there may be a NAPI but it may not have an ID and
   output of a napi_id should be elided.

2. TX-only NAPIs currently do not have NAPI IDs. If a TX queue happens
   to be linked with a TX-only NAPI, elide the NAPI ID from the netlink
   output as a NAPI ID of 0 is not useful for users.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250205193751.297211-1-jdamato@fastly.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/busy_poll.h

@@ -24,6 +24,11 @@
  */
 #define MIN_NAPI_ID ((unsigned int)(NR_CPUS + 1))
 
+static inline bool napi_id_valid(unsigned int napi_id)
+{
+	return napi_id >= MIN_NAPI_ID;
+}
+
 #define BUSY_POLL_BUDGET 8
 
 #ifdef CONFIG_NET_RX_BUSY_POLL

net/core/netdev-genl.c

@@ -365,6 +365,13 @@ int netdev_nl_napi_set_doit(struct sk_buff *skb, struct genl_info *info)
 	return err;
 }
 
+static int nla_put_napi_id(struct sk_buff *skb, const struct napi_struct *napi)
+{
+	if (napi && napi_id_valid(napi->napi_id))
+		return nla_put_u32(skb, NETDEV_A_QUEUE_NAPI_ID, napi->napi_id);
+	return 0;
+}
+
 static int
 netdev_nl_queue_fill_one(struct sk_buff *rsp, struct net_device *netdev,
 			 u32 q_idx, u32 q_type, const struct genl_info *info)
@@ -386,9 +393,7 @@ netdev_nl_queue_fill_one(struct sk_buff *rsp, struct net_device *netdev,
 	switch (q_type) {
 	case NETDEV_QUEUE_TYPE_RX:
 		rxq = __netif_get_rx_queue(netdev, q_idx);
-
-		if (rxq->napi && nla_put_u32(rsp, NETDEV_A_QUEUE_NAPI_ID,
-					     rxq->napi->napi_id))
+		if (nla_put_napi_id(rsp, rxq->napi))
 			goto nla_put_failure;
 
 		params = &rxq->mp_params;
@@ -398,8 +403,7 @@ netdev_nl_queue_fill_one(struct sk_buff *rsp, struct net_device *netdev,
 		break;
 	case NETDEV_QUEUE_TYPE_TX:
 		txq = netdev_get_tx_queue(netdev, q_idx);
-		if (txq->napi && nla_put_u32(rsp, NETDEV_A_QUEUE_NAPI_ID,
-					     txq->napi->napi_id))
+		if (nla_put_napi_id(rsp, txq->napi))
 			goto nla_put_failure;
 	}