[pve-devel] [PATCH qemu-server v3 3/8] migration: preserve host_mtu for virtio-net devices
Fiona Ebner
f.ebner at proxmox.com
Thu Sep 4 14:40:47 CEST 2025
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 at proxmox.com>
---
Changes in v3:
* move get_nets_host_mtu to network module
* avoid overly long line in tests
src/PVE/QemuMigrate.pm | 8 +++++++
src/PVE/QemuServer/Network.pm | 29 +++++++++++++++++++++++
src/test/MigrationTest/QemuMigrateMock.pm | 15 ++++++++++++
3 files changed, 52 insertions(+)
diff --git a/src/PVE/QemuMigrate.pm b/src/PVE/QemuMigrate.pm
index e18cc2aa..4381b542 100644
--- a/src/PVE/QemuMigrate.pm
+++ b/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 = $self->filter_local_volumes('online');
diff --git a/src/PVE/QemuServer/Network.pm b/src/PVE/QemuServer/Network.pm
index 56df83fb..eb8222e8 100644
--- a/src/PVE/QemuServer/Network.pm
+++ b/src/PVE/QemuServer/Network.pm
@@ -11,6 +11,8 @@ use PVE::Network::SDN::Zones;
use PVE::RESTEnvironment qw(log_warn);
use PVE::Tools qw($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;
diff --git a/src/test/MigrationTest/QemuMigrateMock.pm b/src/test/MigrationTest/QemuMigrateMock.pm
index b04cf78b..421f0bb7 100644
--- a/src/test/MigrationTest/QemuMigrateMock.pm
+++ b/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");
--
2.47.2
More information about the pve-devel
mailing list