[BPF] set MTU on bpfin/out.cali to match host

Set the MTU to match the smallest MTU of the host devices as per the
current autodetection. That makes sure that if large MTU is used, the
extra device does not create a bottleneck with a small MTU. Also when
MTU changes, on host devices, it get adjusted automatically.

Fixes projectcalico#8918

felix/dataplane/linux/bpf_ep_mgr.go

@@ -371,6 +371,7 @@ type bpfEndpointManager struct {
 
 	natInIdx  int
 	natOutIdx int
+	hostMTU   int
 
 	v4 *bpfEndpointManagerDataplane
 	v6 *bpfEndpointManagerDataplane
@@ -437,6 +438,7 @@ func NewTestEpMgr(
 		new(environment.FakeFeatureDetector),
 		nil,
 		nil,
+		1500,
 	)
 }
 
@@ -456,6 +458,7 @@ func newBPFEndpointManager(
 	featureDetector environment.FeatureDetectorIface,
 	healthAggregator *health.HealthAggregator,
 	dataplanefeatures *environment.Features,
+	hostMTU int,
 ) (*bpfEndpointManager, error) {
 	if livenessCallback == nil {
 		livenessCallback = func() {}
@@ -623,6 +626,7 @@ func newBPFEndpointManager(
 			}
 		}
 
+		m.hostMTU = hostMTU
 		if err := m.dp.ensureBPFDevices(); err != nil {
 			return nil, fmt.Errorf("ensure BPF devices: %w", err)
 		} else {
@@ -3192,6 +3196,7 @@ func (m *bpfEndpointManager) ensureBPFDevices() error {
 	if err != nil {
 		la := netlink.NewLinkAttrs()
 		la.Name = bpfInDev
+		la.MTU = m.hostMTU
 		nat := &netlink.Veth{
 			LinkAttrs: la,
 			PeerName:  bpfOutDev,
@@ -3204,6 +3209,7 @@ func (m *bpfEndpointManager) ensureBPFDevices() error {
 			return fmt.Errorf("missing %s after add: %w", bpfInDev, err)
 		}
 	}
+
 	if state := bpfin.Attrs().OperState; state != netlink.OperUp {
 		log.WithField("state", state).Info(bpfInDev)
 		if err := netlink.LinkSetUp(bpfin); err != nil {
@@ -3221,6 +3227,15 @@ func (m *bpfEndpointManager) ensureBPFDevices() error {
 		}
 	}
 
+	err = netlink.LinkSetMTU(bpfin, m.hostMTU)
+	if err != nil {
+		return fmt.Errorf("failed to set MTU to %d on %s: %w", m.hostMTU, bpfInDev, err)
+	}
+	err = netlink.LinkSetMTU(bpfout, m.hostMTU)
+	if err != nil {
+		return fmt.Errorf("failed to set MTU to %d on %s: %w", m.hostMTU, bpfOutDev, err)
+	}
+
 	m.natInIdx = bpfin.Attrs().Index
 	m.natOutIdx = bpfout.Attrs().Index
 

felix/dataplane/linux/bpf_ep_mgr_test.go

@@ -455,6 +455,7 @@ var _ = Describe("BPF Endpoint Manager", func() {
 			&environment.FakeFeatureDetector{},
 			nil,
 			environment.NewFeatureDetector(nil).GetFeatures(),
+			1250,
 		)
 		Expect(err).NotTo(HaveOccurred())
 		bpfEpMgr.v4.hostIP = net.ParseIP("1.2.3.4")

felix/dataplane/linux/int_dataplane.go

@@ -716,6 +716,7 @@ func NewIntDataplaneDriver(config Config) *InternalDataplane {
 			featureDetector,
 			config.HealthAggregator,
 			dataplaneFeatures,
+			hostMTU,
 		)
 
 		if err != nil {