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

[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.

[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:
* 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");
-- 
2.39.5