ipsec: Log XFRM errors during temporary state removal

Context: During IPsec upgrades, we may have to temporarily remove some
XFRM states due to conflicts with the new states and because the Linux
API doesn't enable us to perform this atomically as we do for XFRM
policies.

This temporary removal should be very short but can still cause drops
under heavy throughput. This commit logs how many such drops happened.

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>

pkg/datapath/linux/ipsec/ipsec_linux.go

@@ -23,6 +23,7 @@ import (
 	"github.com/cilium/hive/cell"
 	"github.com/cilium/hive/job"
 	"github.com/fsnotify/fsnotify"
+	"github.com/prometheus/procfs"
 	"github.com/sirupsen/logrus"
 	"github.com/vishvananda/netlink"
 
@@ -402,6 +403,18 @@ func xfrmStateReplace(new *netlink.XfrmState, remoteRebooted bool) error {
 // conflicting XFRM state. This function removes the conflicting state and
 // prepares a defer callback to re-add it with proper logging.
 func xfrmTemporarilyRemoveState(scopedLog *logrus.Entry, state netlink.XfrmState, dir string) (error, func()) {
+	stats, err := procfs.NewXfrmStat()
+	errorCnt := 0
+	if err != nil {
+		log.WithError(err).Error("Error while getting XFRM stats before state removal")
+	} else {
+		if dir == "IN" {
+			errorCnt = stats.XfrmInNoStates
+		} else {
+			errorCnt = stats.XfrmOutNoStates
+		}
+	}
+
 	start := time.Now()
 	if err := netlink.XfrmStateDel(&state); err != nil {
 		return err, nil
@@ -411,7 +424,19 @@ func xfrmTemporarilyRemoveState(scopedLog *logrus.Entry, state netlink.XfrmState
 			scopedLog.WithError(err).Errorf("Failed to re-add old XFRM %s state", dir)
 		}
 		elapsed := time.Since(start)
-		scopedLog.WithField(logfields.Duration, elapsed).Infof("Temporarily removed old XFRM %s state", dir)
+
+		stats, err := procfs.NewXfrmStat()
+		if err != nil {
+			log.WithError(err).Error("Error while getting XFRM stats after state removal")
+			errorCnt = 0
+		} else {
+			if dir == "IN" {
+				errorCnt = stats.XfrmInNoStates - errorCnt
+			} else {
+				errorCnt = stats.XfrmOutNoStates - errorCnt
+			}
+		}
+		scopedLog.WithField(logfields.Duration, elapsed).Infof("Temporarily removed old XFRM %s state (%d packets dropped)", dir, errorCnt)
 	}
 }