[PATCH net-next v3] netdev-genl: Elide napi_id when not present

[Joe Damato <jdamato@fastly.com>]

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>
---
 v3:
  - Took Eric's suggested patch to refactor the code to use helpers,
    which improves the code quality significantly.

 v2: https://lore.kernel.org/netdev/20250203191714.155526-1-jdamato@fastly.com/
   - Updated to elide NAPI IDs for RX queues which may have not called
     napi_enable yet.

 rfc: https://lore.kernel.org/lkml/20250128163038.429864-1-jdamato@fastly.com/

 include/net/busy_poll.h |  5 +++++
 net/core/netdev-genl.c  | 13 +++++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index c39a426ebf52..741fa7754700 100644
--- a/include/net/busy_poll.h
+++ b/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
diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c
index 715f85c6b62e..5aa3ed870b2c 100644
--- a/net/core/netdev-genl.c
+++ b/net/core/netdev-genl.c
@@ -364,6 +364,13 @@ 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)
@@ -385,8 +392,7 @@ 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;
 
 		binding = rxq->mp_params.mp_priv;
@@ -397,8 +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, rxq->napi))
 			goto nla_put_failure;
 	}
 

base-commit: c2933b2befe25309f4c5cfbea0ca80909735fd76
-- 
2.43.0