migration: preserve host_mtu for virtio-net devices

The virtual hardware is generated differently (at least for i440fx
machines) when host_mtu is set or not set on the netdev command line
[0]. When the MTU is the same value as the default 1500, Proxmox VE
did not add a host_mtu parameter. This is problematic for migration
where host_mtu is present on one end of the migration, but not on the
other [1]. Moreover, the effective setting in the guest (state) will
still be the host_mtu from the source side, even if a different value
is used for host_mtu on the target instance's commandline. This will
not lead to an error loading the migration stream in QEMU, but having
a larger host_mtu than the bridge MTU is still problematic for certain
network traffic like
> iperf3 -c 10.10.10.11 -u -l 2k
when host_mtu=9000 and bridge MTU=1500.

Pass the values from the source to the target during migration to be
able to preserve them.

[0]: https://bugzilla.redhat.com/show_bug.cgi?id=1449346
[1]: https://forum.proxmox.com/threads/live-vm-migration-fails.169537/post-796379

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Link: https://lore.proxmox.com/20250904124113.81772-4-f.ebner@proxmox.com

src/PVE/QemuMigrate.pm

@@ -998,6 +998,10 @@ sub phase2_start_local_cluster {
         push @$cmd, '--force-cpu', $start->{forcecpu};
     }
 
+    if ($start->{'nets-host-mtu'}) {
+        push @$cmd, '--nets-host-mtu', $start->{'nets-host-mtu'};
+    }
+
     if ($self->{storage_migration}) {
         push @$cmd, '--targetstorage', ($self->{opts}->{targetstorage} // '1');
     }
@@ -1187,6 +1191,10 @@ sub phase2 {
         },
     };
 
+    if (my $nets_host_mtu = PVE::QemuServer::Network::get_nets_host_mtu($vmid, $conf)) {
+        $params->{start_params}->{'nets-host-mtu'} = $nets_host_mtu;
+    }
+
     my ($tunnel_info, $spice_port);
 
     my @online_local_volumes = $self->filter_local_volumes('online');

src/PVE/QemuServer/Network.pm

@@ -11,6 +11,8 @@ use PVE::Network::SDN::Zones;
 use PVE::RESTEnvironment qw(log_warn);
 use PVE::Tools qw($IPV6RE file_read_firstline);
 
+use PVE::QemuServer::Monitor qw(mon_cmd);
+
 my $nic_model_list = [
     'e1000',
     'e1000-82540em',
@@ -330,4 +332,31 @@ sub tap_plug {
     PVE::Network::SDN::Zones::tap_plug($iface, $bridge, $tag, $firewall, $trunks, $rate);
 }
 
+sub get_nets_host_mtu {
+    my ($vmid, $conf) = @_;
+
+    my $nets_host_mtu = [];
+    for my $opt (sort keys $conf->%*) {
+        next if $opt !~ m/^net(\d+)$/;
+        my $net = parse_net($conf->{$opt});
+        next if $net->{model} ne 'virtio';
+
+        my $host_mtu = eval {
+            mon_cmd(
+                $vmid, 'qom-get',
+                path => "/machine/peripheral/$opt",
+                property => 'host_mtu',
+            );
+        };
+        if (my $err = $@) {
+            log_warn("$opt: could not query host_mtu - $err");
+        } elsif (defined($host_mtu)) {
+            push $nets_host_mtu->@*, "${opt}=${host_mtu}";
+        } else {
+            log_warn("$opt: got undefined value when querying host_mtu");
+        }
+    }
+    return join(',', $nets_host_mtu->@*);
+}
+
 1;

src/test/MigrationTest/QemuMigrateMock.pm

@@ -225,6 +225,21 @@ $qemu_server_machine_module->mock(
 my $qemu_server_network_module = Test::MockModule->new("PVE::QemuServer::Network");
 $qemu_server_network_module->mock(
     del_nets_bridge_fdb => sub { return; },
+    mon_cmd => sub {
+        my ($vmid, $command, %params) = @_;
+
+        if ($command eq 'qom-get') {
+            if (
+                $params{path} =~ m|^/machine/peripheral/net\d+$|
+                && $params{property} eq 'host_mtu'
+            ) {
+                return 1500;
+            }
+            die "mon_cmd (mocked) - implement me: $command for path '$params{path}' property"
+                . " '$params{property}'";
+        }
+        die "mon_cmd (mocked) - implement me: $command";
+    },
 );
 
 my $qemu_server_qmphelpers_module = Test::MockModule->new("PVE::QemuServer::QMPHelpers");