net: Don't override default TAP interface MTU

Adjust MTU logic such that:
1. Apply an MTU to the TAP interface if the user supplies it
2. Always query the TAP interface for the MTU and expose that.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/net.rs

@@ -442,10 +442,9 @@ impl Net {
         rate_limiter_config: Option<RateLimiterConfig>,
         exit_evt: EventFd,
     ) -> Result<Self> {
-        let mut mtu = None;
-        if !taps.is_empty() {
-            mtu = Some(taps[0].mtu().map_err(Error::TapError)? as u16);
-        }
+        assert!(!taps.is_empty());
+
+        let mtu = taps[0].mtu().map_err(Error::TapError)? as u16;
 
         let mut avail_features = 1 << VIRTIO_NET_F_CSUM
             | 1 << VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
@@ -458,12 +457,10 @@ impl Net {
             | 1 << VIRTIO_NET_F_HOST_TSO4
             | 1 << VIRTIO_NET_F_HOST_TSO6
             | 1 << VIRTIO_NET_F_HOST_UFO
+            | 1 << VIRTIO_NET_F_MTU
             | 1 << VIRTIO_RING_F_EVENT_IDX
             | 1 << VIRTIO_F_VERSION_1;
 
-        if mtu.is_some() {
-            avail_features |= 1u64 << VIRTIO_NET_F_MTU;
-        }
         if iommu {
             avail_features |= 1u64 << VIRTIO_F_IOMMU_PLATFORM;
         }
@@ -473,9 +470,9 @@ impl Net {
 
         let mut config = VirtioNetConfig::default();
         if let Some(mac) = guest_mac {
-            build_net_config_space(&mut config, mac, num_queues, mtu, &mut avail_features);
+            build_net_config_space(&mut config, mac, num_queues, Some(mtu), &mut avail_features);
         } else {
-            build_net_config_space_with_mq(&mut config, num_queues, mtu, &mut avail_features);
+            build_net_config_space_with_mq(&mut config, num_queues, Some(mtu), &mut avail_features);
         }
 
         Ok(Net {
@@ -545,6 +542,7 @@ impl Net {
         id: String,
         fds: &[RawFd],
         guest_mac: Option<MacAddr>,
+        mtu: Option<u16>,
         iommu: bool,
         queue_size: u16,
         seccomp_action: SeccompAction,
@@ -565,6 +563,12 @@ impl Net {
             taps.push(tap);
         }
 
+        assert!(!taps.is_empty());
+
+        if let Some(mtu) = mtu {
+            taps[0].set_mtu(mtu as i32).map_err(Error::TapError)?;
+        }
+
         Self::new_with_tap(
             id,
             taps,

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -823,7 +823,6 @@ components:
           type: string
         mtu:
           type: integer
-          default: 1280
         iommu:
           type: boolean
           default: false

vmm/src/config.rs

@@ -1247,8 +1247,8 @@ pub struct NetConfig {
     pub mac: MacAddr,
     #[serde(default)]
     pub host_mac: Option<MacAddr>,
-    #[serde(default = "default_netconfig_mtu")]
-    pub mtu: u16,
+    #[serde(default)]
+    pub mtu: Option<u16>,
     #[serde(default)]
     pub iommu: bool,
     #[serde(default = "default_netconfig_num_queues")]
@@ -1286,10 +1286,6 @@ fn default_netconfig_mac() -> MacAddr {
     MacAddr::local_random()
 }
 
-fn default_netconfig_mtu() -> u16 {
-    virtio_devices::net::MIN_MTU
-}
-
 fn default_netconfig_num_queues() -> usize {
     DEFAULT_NUM_QUEUES_VUNET
 }
@@ -1306,7 +1302,7 @@ impl Default for NetConfig {
             mask: default_netconfig_mask(),
             mac: default_netconfig_mac(),
             host_mac: None,
-            mtu: default_netconfig_mtu(),
+            mtu: None,
             iommu: false,
             num_queues: default_netconfig_num_queues(),
             queue_size: default_netconfig_queue_size(),
@@ -1372,8 +1368,7 @@ impl NetConfig {
         let host_mac = parser.convert("host_mac").map_err(Error::ParseNetwork)?;
         let mtu = parser
             .convert("mtu")
-            .map_err(Error::ParseNetwork)?
-            .unwrap_or_else(default_netconfig_mtu);
+            .map_err(Error::ParseNetwork)?;
         let iommu = parser
             .convert::<Toggle>("iommu")
             .map_err(Error::ParseNetwork)?
@@ -1515,8 +1510,10 @@ impl NetConfig {
             }
         }
 
-        if self.mtu < virtio_devices::net::MIN_MTU {
-            return Err(ValidationError::InvalidMtu(self.mtu));
+        if let Some(mtu) = self.mtu {
+            if mtu < virtio_devices::net::MIN_MTU {
+                return Err(ValidationError::InvalidMtu(mtu));
+            }
         }
 
         Ok(())

vmm/src/device_manager.rs

@@ -2244,7 +2244,7 @@ impl DeviceManager {
                 match virtio_devices::vhost_user::Net::new(
                     id.clone(),
                     net_cfg.mac,
-                    Some(net_cfg.mtu),
+                    net_cfg.mtu,
                     vu_cfg,
                     server,
                     self.seccomp_action.clone(),
@@ -2275,7 +2275,7 @@ impl DeviceManager {
                         None,
                         Some(net_cfg.mac),
                         &mut net_cfg.host_mac,
-                        None,
+                        net_cfg.mtu,
                         self.force_iommu | net_cfg.iommu,
                         net_cfg.num_queues,
                         net_cfg.queue_size,
@@ -2293,6 +2293,7 @@ impl DeviceManager {
                         id.clone(),
                         fds,
                         Some(net_cfg.mac),
+                        net_cfg.mtu,
                         self.force_iommu | net_cfg.iommu,
                         net_cfg.queue_size,
                         self.seccomp_action.clone(),
@@ -2312,7 +2313,7 @@ impl DeviceManager {
                         Some(net_cfg.mask),
                         Some(net_cfg.mac),
                         &mut net_cfg.host_mac,
-                        Some(net_cfg.mtu),
+                        net_cfg.mtu,
                         self.force_iommu | net_cfg.iommu,
                         net_cfg.num_queues,
                         net_cfg.queue_size,