[PATCH v3 08/10] net/gve: reject Tx packets that cannot fit in the ring

[Tathagat Priyadarshi]

A TSO packet needing more descriptors than the ring can ever have
free fails the ring space check on every burst and blocks the queue.
One slot is always reserved, the cacheline overlap margin must stay
free, and up to GVE_TX_MIN_RE_INTERVAL - 1 posted descriptors are not
reclaimed until more are posted.

Drop such packets and count them as too_many_descs.

Fixes: 403c671a46b6 ("net/gve: support TSO in DQO RDA")
Cc: stable at dpdk.org

Signed-off-by: Tathagat Priyadarshi <tathagat.dpdk at gmail.com>
---
 drivers/net/gve/gve_tx_dqo.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/net/gve/gve_tx_dqo.c b/drivers/net/gve/gve_tx_dqo.c
index 44d2fa870f..fd36d92fab 100644
--- a/drivers/net/gve/gve_tx_dqo.c
+++ b/drivers/net/gve/gve_tx_dqo.c
@@ -126,14 +126,30 @@ gve_tx_clean_descs_dqo(struct gve_tx_queue *txq, uint16_t nb_descs) {
 		gve_tx_clean_dqo(txq);
 }
 
+/* Max descriptors a single packet may use. A larger packet can never pass
+ * gve_can_tx(): one slot is always reserved, the cacheline overlap margin
+ * must stay free, and up to GVE_TX_MIN_RE_INTERVAL - 1 descriptors posted
+ * after the last report_event are not reclaimed until more are posted.
+ */
+static inline int
+gve_tx_max_pkt_descs(const struct gve_tx_queue *txq)
+{
+	return (int)txq->nb_tx_desc - 1 - GVE_TX_MIN_DESC_PREVENT_CACHE_OVERLAP -
+		(GVE_TX_MIN_RE_INTERVAL - 1);
+}
+
 /* GVE expects at most 10 data descriptors per mtu-sized segment. Beyond this,
  * the hardware will assume the driver is malicious and stop transmitting
  * packets altogether. Validate that a packet can be sent to avoid sending
  * posting descriptors for an invalid packet.
  */
 static inline bool
-gve_tx_validate_descs(struct rte_mbuf *tx_pkt, uint16_t nb_descs, bool is_tso)
+gve_tx_validate_descs(const struct gve_tx_queue *txq, struct rte_mbuf *tx_pkt,
+		      uint16_t nb_descs, bool is_tso)
 {
+	if (nb_descs > gve_tx_max_pkt_descs(txq))
+		return false;
+
 	if (!is_tso)
 		return nb_descs <= GVE_TX_MAX_DATA_DESCS;
 
@@ -297,7 +313,7 @@ gve_tx_burst_dqo(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
 		nb_descs += tso;
 
 		/* Drop packet if it doesn't adhere to hardware limits. */
-		if (!gve_tx_validate_descs(tx_pkt, nb_descs, tso)) {
+		if (!gve_tx_validate_descs(txq, tx_pkt, nb_descs, tso)) {
 			txq->stats.too_many_descs++;
 			rte_pktmbuf_free(tx_pkt);
 			continue;
-- 
2.43.0