snapshot: introduce running-nets-host-mtu property

For VirtIO network devices, it is necessary to preserve the values and
presence of the host_mtu setting when restoring a snapshot. See commit
"migration: preserve host_mtu for virtio-net devices" for details.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Link: https://lore.proxmox.com/20250904124113.81772-7-f.ebner@proxmox.com

src/PVE/API2/Qemu.pm

@@ -2113,6 +2113,7 @@ my $update_vm_api = sub {
             }
             push @delete, 'runningmachine' if $conf->{runningmachine};
             push @delete, 'runningcpu' if $conf->{runningcpu};
+            push @delete, 'running-nets-host-mtu' if $conf->{'running-nets-host-mtu'};
         }
 
         PVE::QemuConfig->check_lock($conf) if !$skiplock;

src/PVE/QemuConfig.pm

@@ -247,6 +247,8 @@ sub __snapshot_save_vmstate {
         or die "cannot obtain PID for VM $vmid!\n";
     my $runningcpu = PVE::QemuServer::CPUConfig::get_cpu_from_running_vm($pid);
 
+    my $nets_host_mtu = PVE::QemuServer::Network::get_nets_host_mtu($vmid, $conf);
+
     if (!$suspend) {
         $conf = $conf->{snapshots}->{$snapname};
     }
@@ -254,6 +256,7 @@ sub __snapshot_save_vmstate {
     $conf->{vmstate} = $statefile;
     $conf->{runningmachine} = $runningmachine;
     $conf->{runningcpu} = $runningcpu;
+    $conf->{'running-nets-host-mtu'} = $nets_host_mtu;
 
     return $statefile;
 }
@@ -473,6 +476,8 @@ sub __snapshot_rollback_hook {
             # re-initializing its random number generator
             $conf->{vmgenid} = PVE::QemuServer::generate_uuid();
         }
+
+        $data->{'nets-host-mtu'} = delete($conf->{'running-nets-host-mtu'});
     }
 
     return;
@@ -513,6 +518,7 @@ sub __snapshot_rollback_vm_start {
         statefile => $vmstate,
         forcemachine => $data->{forcemachine},
         forcecpu => $data->{forcecpu},
+        'nets-host-mtu' => $data->{'nets-host-mtu'},
     };
     PVE::QemuServer::vm_start($storecfg, $vmid, $params);
 }

src/PVE/QemuServer.pm

@@ -635,6 +635,15 @@ EODESCR
         pattern => $PVE::QemuServer::CPUConfig::qemu_cmdline_cpu_re,
         format_description => 'QEMU -cpu parameter',
     },
+    'running-nets-host-mtu' => {
+        type => 'string',
+        pattern => 'net\d+=\d+(,net\d+=\d+)*',
+        optional => 1,
+        description =>
+            'List of VirtIO network devices and their effective host_mtu setting. A value of 0'
+            . ' means that the host_mtu parameter is to be avoided for the corresponding device.'
+            . ' This is used internally for snapshots.',
+    },
     machine => get_standard_option('pve-qemu-machine'),
     arch => {
         description => "Virtual processor architecture. Defaults to the host.",

src/PVE/QemuServer/RunState.pm

@@ -104,7 +104,8 @@ sub vm_suspend {
                         warn $@ if $@;
                         PVE::Storage::deactivate_volumes($storecfg, [$vmstate]);
                         PVE::Storage::vdisk_free($storecfg, $vmstate);
-                        delete $conf->@{qw(vmstate runningmachine runningcpu)};
+                        delete $conf->@{
+                            qw(vmstate runningmachine runningcpu running-nets-host-mtu)};
                         PVE::QemuConfig->write_config($vmid, $conf);
                     };
                     warn $@ if $@;