Re: [pve-devel] [PATCH qemu-server v3 stable-bookworm 8/8] migration: preserve host_mtu for virtio-net devices

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

Am 04.09.25 um 14:42 schrieb Fiona Ebner:
> 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.

Which breaks migration from new to old, which can be fine, but seems
avoidable given that we got a tunnel that we can query stuff over.

Maybe we could at least catch the "Unknown option: nets-host-mtu"
error explicitly and add some context that the target likely just
needs to be updated to make the migration work.

> 
> [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>
> ---
> 
> Changes in v3:
> * Adapt to moved/changed get_nets_host_mtu() helper. The Network
>   module does not exist yet in PVE 8, and parse_net() lives in
>   QemuServer itself, so the helper is also moved there.
> 
>  src/PVE/QemuMigrate.pm                    |  8 +++++++
>  src/PVE/QemuServer.pm                     | 27 +++++++++++++++++++++++
>  src/test/MigrationTest/QemuMigrateMock.pm | 15 +++++++++++++
>  3 files changed, 50 insertions(+)
> 
> diff --git a/src/PVE/QemuMigrate.pm b/src/PVE/QemuMigrate.pm
> index 28d7ac56..3cd7069a 100644
> --- a/src/PVE/QemuMigrate.pm
> +++ b/src/PVE/QemuMigrate.pm
> @@ -959,6 +959,10 @@ 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 +1148,10 @@ sub phase2 {
>          },
>      };
>  
> +    if (my $nets_host_mtu = PVE::QemuServer::get_nets_host_mtu($vmid, $conf)) {
> +        $params->{start_params}->{'nets-host-mtu'} = $nets_host_mtu;
> +    }
> +
>      my ($tunnel_info, $spice_port);
>  
>      my @online_local_volumes = $self->filter_local_volumes('online');
> diff --git a/src/PVE/QemuServer.pm b/src/PVE/QemuServer.pm
> index 47c96726..5b08b3e3 100644
> --- a/src/PVE/QemuServer.pm
> +++ b/src/PVE/QemuServer.pm
> @@ -9533,4 +9533,31 @@ sub delete_ifaces_ipams_ips {
>      }
>  }
>  
> +sub get_nets_host_mtu {
> +    my ($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 f678f9ec..05b2c5c1 100644
> --- a/src/test/MigrationTest/QemuMigrateMock.pm
> +++ b/src/test/MigrationTest/QemuMigrateMock.pm
> @@ -175,6 +175,21 @@ $MigrationTest::Shared::qemu_server_module->mock(
>          delete $expected_calls->{'vm_stop'};
>      },
>      del_nets_bridge_fdb => sub { return; },
> +    mon_cmd => sub {
> +        my ($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_cpuconfig_module = Test::MockModule->new("PVE::QemuServer::CPUConfig");



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