Re: [pve-devel] [PATCH qemu-server v2 stable-bookworm 4/4] migration: preserve host_mtu for virtio-net devices

["Fabian Grünbichler" <f.gruenbichler@proxmox.com>]

On September 3, 2025 4:22 pm, Fiona Ebner wrote:
> 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.

since the effective actual MTU value is determined by the migration
state, this means 8->8 migrations potentially also break traffic:

VM config: mtu=1 (use bridge MTU)
source node: bridge MTU 9000
host_mtu for starting source VM: 9000

target node: bridge MTU 1500
host_mtu for starting target VM: 1500
effective MTU of the vNIC: 9000, because that's what the source VM's
migration stream tells the target OS?

right? so it might make sense to still detect and handle this also on
8.x? can be done as a follow-up of course..

> 
> Pass the values from the source to the target during migration to be
> able to preserve them.
> 
> While the setting only takes effect starting with Proxmox VE 9, it is
> necessary to add a dummy parameter to the VM start API already in
> Proxmox VE 8.
> 
> [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>
> ---
>  src/PVE/API2/Qemu.pm                      |  6 ++++
>  src/PVE/QemuMigrate.pm                    | 37 +++++++++++++++++++++++
>  src/test/MigrationTest/QemuMigrateMock.pm |  9 ++++++
>  3 files changed, 52 insertions(+)
> 
> diff --git a/src/PVE/API2/Qemu.pm b/src/PVE/API2/Qemu.pm
> index ce6f362d..d15dbc2e 100644
> --- a/src/PVE/API2/Qemu.pm
> +++ b/src/PVE/API2/Qemu.pm
> @@ -3364,6 +3364,12 @@ __PACKAGE__->register_method({
>                  default => 'max(30, vm memory in GiB)',
>                  optional => 1,
>              },
> +            'nets-host-mtu' => {
> +                type => 'string',
> +                pattern => 'net\d+=\d+(,net\d+=\d+)*',
> +                optional => 1,
> +                description => "Compat only. Parameter is only used starting with Proxmox VE 9.",
> +            },
>          },
>      },
>      returns => {
> diff --git a/src/PVE/QemuMigrate.pm b/src/PVE/QemuMigrate.pm
> index 28d7ac56..f8237692 100644
> --- a/src/PVE/QemuMigrate.pm
> +++ b/src/PVE/QemuMigrate.pm
> @@ -170,6 +170,35 @@ sub target_storage_check_available {
>      }
>  }
>  
> +my sub get_nets_host_mtu {
> +    my ($self, $vmid) = @_;
> +
> +    my $conf = $self->{vmconf};
> +
> +    my $nets_host_mtu = [];
> +    for my $opt (sort keys $conf->%*) {
> +        next if $opt !~ m/^net(\d+)$/;
> +        my $net = PVE::QemuServer::parse_net($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 = $@) {
> +            $self->log('warn', "$opt: could not query host_mtu - $err");
> +        } elsif (defined($host_mtu)) {
> +            push $nets_host_mtu->@*, "${opt}=${host_mtu}";
> +        } else {
> +            $self->log('warn', "$opt: got undefined value when querying host_mtu");
> +        }
> +    }
> +    return join(',', $nets_host_mtu->@*);
> +}
> +
>  sub prepare {
>      my ($self, $vmid) = @_;
>  
> @@ -959,6 +988,10 @@ sub phase2_start_local_cluster {
>          push @$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');
>      }
> @@ -1144,6 +1177,10 @@ sub phase2 {
>          },
>      };
>  
> +    if (my $nets_host_mtu = get_nets_host_mtu($self, $vmid)) {
> +        $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/test/MigrationTest/QemuMigrateMock.pm b/src/test/MigrationTest/QemuMigrateMock.pm
> index f678f9ec..18735ed6 100644
> --- a/src/test/MigrationTest/QemuMigrateMock.pm
> +++ b/src/test/MigrationTest/QemuMigrateMock.pm
> @@ -111,6 +111,15 @@ $qemu_migrate_module->mock(
>              return;
>          } elsif ($command eq 'migrate_cancel') {
>              return;
> +        } elsif ($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";
>      },
> -- 
> 2.39.5
> 
> 
> 
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> 
> 


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