Read rx errors from the driver stats array and add the pcs counters

main.go

@@ -48,6 +48,7 @@ type direction struct {
 	probeSpec *frameSpec
 	probeTxFD int
 	probeRxFD int
+	statFD    int
 	cable     *cableStats
 
 	// Guards everything the sampler touches. The counters are read on their own
@@ -456,7 +457,18 @@ func buildDirection(label string, tx, rx endpoint, sizes []int, cfg config) (*di
 		streams: newLossWindows(cfg.streams),
 		cable:   newCableStats(),
 	}
-	d.poller = &nicPoller{txName: tx.name, rxName: rx.name, total: &d.nic}
+	// Held open for the life of the run: the stats ioctl is issued five times a
+	// second and reopening a socket for each one is pure overhead.
+	statFD, err := unix.Socket(unix.AF_INET, unix.SOCK_DGRAM, 0)
+	if err != nil {
+		return nil, fmt.Errorf("%s stats socket: %w", label, err)
+	}
+	d.statFD = statFD
+
+	d.poller, err = newNICPoller(statFD, tx.name, rx.name, &d.nic)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", label, err)
+	}
 	d.win = newRateWindow(int(rateWindowSpan/sampleInterval) + 1)
 
 	for i := 0; i < cfg.streams; i++ {
@@ -563,6 +575,7 @@ func (d *direction) close() {
 	}
 	unix.Close(d.probeTxFD)
 	unix.Close(d.probeRxFD)
+	unix.Close(d.statFD)
 }
 
 type config struct {