Re: [PATCH] hw/core/machine: move compatibility flags for VirtIO-net USO to machine 8.1

[Fabiano Rosas]

Fiona Ebner <f.ebner@proxmox.com> writes:

> Migration from an 8.2 or 9.0 binary to an 8.1 binary with machine
> version 8.1 can fail with:
>
>> kvm: Features 0x1c0010130afffa7 unsupported. Allowed features: 0x10179bfffe7
>> kvm: Failed to load virtio-net:virtio
>> kvm: error while loading state for instance 0x0 of device 
>> '0000:00:12.0/virtio-net'
>> kvm: load of migration failed: Operation not permitted
>
> The series
>
> 53da8b5a99 virtio-net: Add support for USO features
> 9da1684954 virtio-net: Add USO flags to vhost support.
> f03e0cf63b tap: Add check for USO features
> 2ab0ec3121 tap: Add USO support to tap device.
>
> only landed in QEMU 8.2, so the compatibility flags should be part of
> machine version 8.1.
>
> Moving the flags unfortunately breaks forward migration with machine
> version 8.1 from a binary without this patch to a binary with this
> patch.
>
> Fixes: 53da8b5a99 ("virtio-net: Add support for USO features")
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

Reviewed-by: Fabiano Rosas <farosas@suse.de>

I'll get to it eventually, but is this another one where just having
-device virtio-net in the command line when testing cross-version
migration would already have caught the issue?