[PATCH 3/3] virtio-net: add mtu-from-tap property for automatic MTU detection

[Nariman Sayed <narimansayed28@gmail.com>]

Currently, exposing the host tap device's MTU to the guest via
VIRTIO_NET_F_MTU requires manually specifying host_mtu= on the
virtio-net-pci device, duplicating the MTU already configured on
the host's tap interface.

Add a new mtu-from-tap boolean property (default off, preserving
existing behavior). When enabled and the device's netdev peer is
a tap client, the host tap's MTU is queried via tap_get_mtu() and
used to set net_conf.mtu and enable VIRTIO_NET_F_MTU.

This detection must happen in virtio_net_device_realize(), before
virtio_net_set_config_size() computes the virtio config space
size from host_features. Performing the detection later (e.g. in
the get_features_ex() callback, which runs after the NIC and its
config space are already sized) results in the mtu config field
falling outside the sized config space, and the guest reading
back an unmapped garbage value (0xffff) instead of the intended
MTU.

Since qemu_new_nic() has not yet been called at this point in
realize(), the tap peer is accessed via n->nic_conf.peers.ncs[0]
(populated during property parsing, before realize() runs, via
the netdev= property) rather than through n->nic.

Signed-off-by: Nariman Sayed <narimansayed28@gmail.com>
---
 hw/net/virtio-net.c            | 12 ++++++++++++
 include/hw/virtio/virtio-net.h |  1 +
 2 files changed, 13 insertions(+)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 814b99a43d..6faeca87d4 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -3901,6 +3901,17 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
     }
 
+    if (n->net_conf.mtu_from_tap) {
+        NetClientState *tap_peer = n->nic_conf.peers.ncs[0];
+        if (tap_peer && tap_peer->info->type == NET_CLIENT_DRIVER_TAP) {
+            int mtu = tap_get_mtu(tap_peer);
+            if (mtu > 0) {
+                n->net_conf.mtu = mtu;
+                n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
+            }
+        }
+    }
+
     if (n->net_conf.duplex_str) {
         if (strncmp(n->net_conf.duplex_str, "half", 5) == 0) {
             n->net_conf.duplex = DUPLEX_HALF;
@@ -4275,6 +4286,7 @@ static const Property virtio_net_properties[] = {
     DEFINE_PROP_UINT16("tx_queue_size", VirtIONet, net_conf.tx_queue_size,
                        VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE),
     DEFINE_PROP_UINT16("host_mtu", VirtIONet, net_conf.mtu, 0),
+    DEFINE_PROP_BOOL("mtu-from-tap", VirtIONet, net_conf.mtu_from_tap, false),
     DEFINE_PROP_INT32("speed", VirtIONet, net_conf.speed, SPEED_UNKNOWN),
     DEFINE_PROP_STRING("duplex", VirtIONet, net_conf.duplex_str),
     DEFINE_PROP_BOOL("failover", VirtIONet, failover, false),
diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
index 371e376428..11044b6d27 100644
--- a/include/hw/virtio/virtio-net.h
+++ b/include/hw/virtio/virtio-net.h
@@ -52,6 +52,7 @@ typedef struct virtio_net_conf
     uint16_t rx_queue_size;
     uint16_t tx_queue_size;
     uint16_t mtu;
+    bool mtu_from_tap;
     int32_t speed;
     char *duplex_str;
     uint8_t duplex;
-- 
2.34.1