Migration MTU Problems with fixed MTU 1500 with machine version <10.1

[fstrankowski]

I recently came across the following warning while migrating a few VMs within our cluster.

netdev net0: not adding 'host_mtu' parameter for migration compat
WARN: netdev net0: ignoring MTU '1500', not used on the source side according to migration parameters
/dev/rbd1
migration listens on unix:/run/qemu-server/3006.migrate

TASK WARNINGS: 1

Upon reviewing qemu-server.pm in combination with this commit it came to my attention that this warning is yield when the following parameters are met:

• VM with VirtIO networking driver
• $migration_skip_host_mtu is true which is $nets_host_mtu->{$netname}

This happens if a VM with defined MTU of 1500 is migrated - with a running machine version < 10.1.

E.g.:

pvesh get nodes/HOST-NAME-123/qemu/4010/status/current

┌────────────────────┬─────────────────────────────────┐
│ key                │ value                           │
╞════════════════════╪═════════════════════════════════╡
│[...]
├────────────────────┼─────────────────────────────────┤
│ running-machine    │ pc-q35-9.1+pve0                 │
├────────────────────┼─────────────────────────────────┤
│[...]

It seems that net0=0 is wrongly assumed when an MTU of 1500 is set.
Any ideas?