[pve-devel] [PATCH qemu-server v2 3/4] migration: preserve host_mtu for virtio-net devices

[Fiona Ebner <f.ebner@proxmox.com>]

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@proxmox.com>
---
 src/PVE/QemuMigrate.pm                    | 37 +++++++++++++++++++++++
 src/test/MigrationTest/QemuMigrateMock.pm |  9 ++++++
 2 files changed, 46 insertions(+)

diff --git a/src/PVE/QemuMigrate.pm b/src/PVE/QemuMigrate.pm
index e18cc2aa..a43bd615 100644
--- a/src/PVE/QemuMigrate.pm
+++ b/src/PVE/QemuMigrate.pm
@@ -175,6 +175,35 @@ 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 $net = PVE::QemuServer::Network::parse_net($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) = @_;
 
@@ -998,6 +1027,10 @@ 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 +1220,10 @@ 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 b04cf78b..ca88d758 100644
--- a/src/test/MigrationTest/QemuMigrateMock.pm
+++ b/src/test/MigrationTest/QemuMigrateMock.pm
@@ -112,6 +112,15 @@ $qemu_migrate_module->mock(
             return;
         } elsif ($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";
     },
-- 
2.47.2



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