vpp: T9161: Enforce NIC MTU limit and apply MTU to VLAN sub-interfaces

On a VPP-bound interface the MTU limit is the NIC's, which only VPP knows
(the kernel side is an LCP tap reporting a meaningless larger max). Reject at
commit an MTU larger than that limit, read from VPP's 'show hardware-interfaces'
(max mtu + driver frame overhead), so the kernel and dataplane cannot silently
end up with different MTUs.

Also apply a VLAN sub-interface's MTU to its VPP dataplane sub-interface and its
linux-cp tap (created with MTU 0), not just the parent; a sub-interface without
its own MTU inherits the parent's. Covers ethernet (vif/vif-s) and VPP
bond/loopback (vif).

4 files changed, 87 insertions, 19 deletions

diff --git a/python/vyos/ifconfig/vpp/bond.py b/python/vyos/ifconfig/vpp/bond.py
index 240657383..095cfc538 100644
--- a/python/vyos/ifconfig/vpp/bond.py
+++ b/python/vyos/ifconfig/vpp/bond.py
@@ -160,8 +160,11 @@ class VPPBondInterface(Interface, VPPInterface):
             if member not in members:
                 self.add_member(interface=member)
 
+        # Apply all settings to the lcp pair (kernel) interface first: creating
+        # the kernel VLANs is what makes linux-cp create the matching VPP
+        # sub-interfaces and their taps, which the VPP-specific settings below
+        # (MTU sync to those taps) then depend on.
+        super().update(config)
+
         # Apply VPP-specific interface settings
         VPPInterface.update(self, config)
-
-        # Apply all settings to the lcp pair (kernel) interface
-        super().update(config)
diff --git a/python/vyos/ifconfig/vpp/interface.py b/python/vyos/ifconfig/vpp/interface.py
index 17be83814..99ade51d2 100644
--- a/python/vyos/ifconfig/vpp/interface.py
+++ b/python/vyos/ifconfig/vpp/interface.py
@@ -55,22 +55,44 @@ class VPPInterface:
             if lcp_name:
                 self.vpp.iface_rxmode(lcp_name, rx_mode)
 
-    def set_mtu_vpp(self, mtu):
-        # Set MTU for the VPP interface
-        self.vpp.set_iface_mtu(self.vpp_ifname, mtu)
+    def set_mtu_vpp(self, config):
+        """Set the MTU on this interface and its VLAN sub-interfaces.
 
-        # Set MTU for the LCP pair interface
-        lcp_pair = self.vpp.lcp_pair_find(vpp_name_hw=self.vpp_ifname)
-        if lcp_pair:
-            lcp_name = lcp_pair.get('vpp_name_kernel')
-            if lcp_name:
-                self.vpp.set_iface_mtu(lcp_name, mtu)
+        The MTU is set on both the dataplane interface and its LCP tap. The
+        tap is created with MTU 0, so it always needs setting. A VLAN
+        sub-interface uses its own MTU if set, otherwise it inherits the
+        parent MTU.
+        """
+        parent_mtu = config.get('mtu')
+        if not parent_mtu:
+            return
+
+        # Fetch all LCP pairs
+        lcp_tap_map = {
+            pair.get('vpp_name_hw'): pair.get('vpp_name_kernel')
+            for pair in self.vpp.lcp_pairs_list()
+        }
+
+        # Parent: always set the dataplane interface, plus its tap if present.
+        self.vpp.set_iface_mtu(self.vpp_ifname, int(parent_mtu))
+        parent_tap = lcp_tap_map.get(self.vpp_ifname)
+        if parent_tap:
+            self.vpp.set_iface_mtu(parent_tap, int(parent_mtu))
+
+        # VLAN sub-interfaces: set the sub-interface and its tap once linux-cp
+        # has created the tap.
+        for vlan_id, vlan_config in config.get('vif', {}).items():
+            vlan_ifname = f'{self.vpp_ifname}.{vlan_id}'
+            lcp_name = lcp_tap_map.get(vlan_ifname)
+            if not lcp_name:
+                continue
+            mtu = int(vlan_config.get('mtu') or parent_mtu)
+            self.vpp.set_iface_mtu(vlan_ifname, mtu)
+            self.vpp.set_iface_mtu(lcp_name, mtu)
 
     def update(self, config):
-        # Set MTU
-        if 'mtu' in config:
-            mtu = int(config['mtu'])
-            self.set_mtu_vpp(mtu)
+        # Set MTU on the interface, its VLAN sub-interfaces, and their LCP taps
+        self.set_mtu_vpp(config)
 
         # Set rx-mode
         rx_mode = config.get('vpp_settings', {}).get('interface_rx_mode')
diff --git a/python/vyos/ifconfig/vpp/loopback.py b/python/vyos/ifconfig/vpp/loopback.py
index 9f87781c0..ee34a5348 100644
--- a/python/vyos/ifconfig/vpp/loopback.py
+++ b/python/vyos/ifconfig/vpp/loopback.py
@@ -106,7 +106,16 @@ class VPPLoopbackInterface(Interface, VPPInterface):
         # Add loopback interface
         self.add()
 
-        VPPInterface.update(self, config)
-
-        # Apply all settings to the lcp pair (kernel) interface
+        # Apply all settings to the lcp pair (kernel) interface first: creating
+        # the kernel VLANs is what makes linux-cp create the matching VPP
+        # sub-interfaces and their taps, which the VPP-specific settings below
+        # (MTU sync to those taps) then depend on.
         super().update(config)
+
+        # Without a configured MTU the loopback keeps VPP's default; take it
+        # from the kernel so it still reaches the LCP taps and VLAN
+        # sub-interfaces. An explicit MTU is applied by VPPInterface.
+        if 'mtu' not in config:
+            config['mtu'] = self.get_mtu()
+
+        VPPInterface.update(self, config)
diff --git a/python/vyos/vpp/control_vpp.py b/python/vyos/vpp/control_vpp.py
index 5cad2a646..49c55b56e 100644
--- a/python/vyos/vpp/control_vpp.py
+++ b/python/vyos/vpp/control_vpp.py
@@ -535,6 +535,40 @@ class VPPControl:
         return self.__vpp_api_client.api.sw_interface_set_mtu(**api_call_args)
 
     @_Decorators.api_call
+    def get_iface_max_mtu(self, iface_name_vpp: str) -> int | None:
+        """Return the maximum L3 MTU an interface's NIC can accept.
+
+        Derived from the driver limits in 'show hardware-interfaces': 'max mtu'
+        (rte_eth_dev_info.max_mtu) plus 'driver frame overhead'. The NIC is owned
+        by the VPP dataplane, so only VPP knows its real limit - the kernel side
+        is an LCP tap that reports its own, much larger max MTU and does not
+        reflect the physical NIC.
+
+        Args:
+            iface_name_vpp (str): Name of an interface in VPP
+
+        Returns:
+            int | None: maximum L3 MTU or None
+        """
+        hw_info = self.cli_cmd(f'show hardware-interfaces {iface_name_vpp}').reply or ''
+        max_mtu = re.search(r'max mtu:\s*(\d+)', hw_info)
+        if not max_mtu:
+            return None
+
+        max_mtu = int(max_mtu.group(1))
+        overhead = re.search(r'driver frame overhead:\s*(\d+)', hw_info)
+        if not overhead:
+            # Without the driver frame overhead the L3 limit is unknown.
+            return None
+
+        # VPP programs the NIC with (L3 MTU + hi->frame_overhead -
+        # driver_frame_overhead); hi->frame_overhead for a DPDK Ethernet
+        # interface is Ethernet header (14) + two QinQ VLAN tags (8) = 22. So the
+        # largest settable L3 MTU is (max_mtu + driver_frame_overhead - 22).
+        l2_frame_overhead = 14 + 8
+        return max_mtu + int(overhead.group(1)) - l2_frame_overhead
+
+    @_Decorators.api_call
     def get_sw_if_dev_type(self, ifname: str) -> int | None:
         """Find interface device type by interface name in VPP