i40e: report VF tx_dropped with tx_errors instead of tx_discards

Currently the tx_dropped field in VF stats is not updated correctly
when reading stats from the PF. This is because it reads from
i40e_eth_stats.tx_discards which seems to be unused for per VSI stats,
as it is not updated by i40e_update_eth_stats() and the corresponding
register, GLV_TDPC, is not implemented[1].

Use i40e_eth_stats.tx_errors instead, which is actually updated by
i40e_update_eth_stats() by reading from GLV_TEPC.

To test, create a VF and try to send bad packets through it:

$ echo 1 > /sys/class/net/enp2s0f0/device/sriov_numvfs
$ cat test.py
from scapy.all import *

vlan_pkt = Ether(dst="ff:ff:ff:ff:ff:ff") / Dot1Q(vlan=999) / IP(dst="192.168.0.1") / ICMP()
ttl_pkt = IP(dst="8.8.8.8", ttl=0) / ICMP()

print("Send packet with bad VLAN tag")
sendp(vlan_pkt, iface="enp2s0f0v0")
print("Send packet with TTL=0")
sendp(ttl_pkt, iface="enp2s0f0v0")
$ ip -s link show dev enp2s0f0
16: enp2s0f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether 3c:ec:ef:b7:e0:ac brd ff:ff:ff:ff:ff:ff
    RX:  bytes packets errors dropped  missed   mcast
             0       0      0       0       0       0
    TX:  bytes packets errors dropped carrier collsns
             0       0      0       0       0       0
    vf 0     link/ether e2:c6:fd:c1:1e:92 brd ff:ff:ff:ff:ff:ff, spoof checking on, link-state auto, trust off
    RX: bytes  packets  mcast   bcast   dropped
             0        0       0       0        0
    TX: bytes  packets   dropped
             0        0        0
$ python test.py
Send packet with bad VLAN tag
.
Sent 1 packets.
Send packet with TTL=0
.
Sent 1 packets.
$ ip -s link show dev enp2s0f0
16: enp2s0f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether 3c:ec:ef:b7:e0:ac brd ff:ff:ff:ff:ff:ff
    RX:  bytes packets errors dropped  missed   mcast
             0       0      0       0       0       0
    TX:  bytes packets errors dropped carrier collsns
             0       0      0       0       0       0
    vf 0     link/ether e2:c6:fd:c1:1e:92 brd ff:ff:ff:ff:ff:ff, spoof checking on, link-state auto, trust off
    RX: bytes  packets  mcast   bcast   dropped
             0        0       0       0        0
    TX: bytes  packets   dropped
             0        0        0

A packet with non-existent VLAN tag and a packet with TTL = 0 are sent,
but tx_dropped is not incremented.

After patch:

$ ip -s link show dev enp2s0f0
19: enp2s0f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether 3c:ec:ef:b7:e0:ac brd ff:ff:ff:ff:ff:ff
    RX:  bytes packets errors dropped  missed   mcast
             0       0      0       0       0       0
    TX:  bytes packets errors dropped carrier collsns
             0       0      0       0       0       0
    vf 0     link/ether 4a:b7:3d:37:f7:56 brd ff:ff:ff:ff:ff:ff, spoof checking on, link-state auto, trust off
    RX: bytes  packets  mcast   bcast   dropped
             0        0       0       0        0
    TX: bytes  packets   dropped
             0        0        2

Fixes: dc645da ("i40e: implement VF stats NDO")
Signed-off-by: Dennis Chen <[email protected]>
Link: https://www.intel.com/content/www/us/en/content-details/596333/intel-ethernet-controller-x710-tm4-at2-carlsville-datasheet.html
Reviewed-by: Simon Horman <[email protected]>
Tested-by: Rafal Romanowski <[email protected]>
Signed-off-by: Tony Nguyen <[email protected]>

Author: dchen5022

drivers/net/ethernet/intel/i40e/i40e_virtchnl_pf.c

@@ -5006,7 +5006,7 @@ int i40e_get_vf_stats(struct net_device *netdev, int vf_id,
 	vf_stats->broadcast  = stats->rx_broadcast;
 	vf_stats->multicast  = stats->rx_multicast;
 	vf_stats->rx_dropped = stats->rx_discards + stats->rx_discards_other;
-	vf_stats->tx_dropped = stats->tx_discards;
+	vf_stats->tx_dropped = stats->tx_errors;
 
 	return 0;
 }