[PATCH 2/3] net/ice: fix DCF Rx packets statistics underflow

[Ciara Loftus <ciara.loftus@intel.com>]

The number of Rx packets is computed as the sum of the unicast,
multicast and broadcast packet counters, minus the discarded packet
count:

    ipackets = rx_unicast + rx_multicast + rx_broadcast - rx_discards

The unicast, multicast and broadcast counters already include the
packets that were subsequently dropped, so subtracting rx_discards
yields only the packets delivered to the application. These values are
provided by the PF in a virtchnl_eth_stats message; the PF samples them
from separate sources and the order in which they are read cannot be
guaranteed. Under load, rx_discards can therefore momentarily exceed the
sum of the unicast, multicast and broadcast counters. As ipackets is
unsigned, the subtraction then wraps to a huge bogus value, reported to
the application as an enormous Rx packet count and packet rate.

The read order cannot be guaranteed, so use a saturating subtraction:
when rx_discards exceeds the sum of the unicast, multicast and broadcast
counters essentially nothing was delivered, so report zero instead of
underflowing.

Fixes: c9f889e99616 ("net/ice: enable stats for DCF")
Cc: stable@dpdk.org

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
---
 drivers/net/intel/ice/ice_dcf_ethdev.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/net/intel/ice/ice_dcf_ethdev.c b/drivers/net/intel/ice/ice_dcf_ethdev.c
index 4fce59617e..c78b290b0d 100644
--- a/drivers/net/intel/ice/ice_dcf_ethdev.c
+++ b/drivers/net/intel/ice/ice_dcf_ethdev.c
@@ -1523,8 +1523,16 @@ ice_dcf_stats_get(struct rte_eth_dev *dev, struct rte_eth_stats *stats,
 	ret = ice_dcf_query_stats(hw, &pstats);
 	if (ret == 0) {
 		ice_dcf_update_stats(&hw->eth_stats_offset, &pstats);
-		stats->ipackets = pstats.rx_unicast + pstats.rx_multicast +
-				pstats.rx_broadcast - pstats.rx_discards;
+		stats->ipackets = pstats.rx_unicast + pstats.rx_multicast + pstats.rx_broadcast;
+		/*
+		 * Unicast/multicast/broadcast counters include discarded packets, so subtract
+		 * rx_discards to report only the packets delivered to the application. The
+		 * counters are sampled from separate sources and can be momentarily inconsistent
+		 * under load. If rx_discards exceeds their sum then essentially nothing was
+		 * delivered, so saturate at zero rather than underflow.
+		 */
+		stats->ipackets = stats->ipackets >= pstats.rx_discards ?
+					stats->ipackets - pstats.rx_discards : 0;
 		stats->opackets = pstats.tx_broadcast + pstats.tx_multicast +
 						pstats.tx_unicast;
 		stats->imissed = pstats.rx_discards;
-- 
2.43.0