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

[Fiona Ebner]

Am 21.05.24 um 00:22 schrieb 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?
> 
AFAIU, the guest kernel needs to be recent enough to support the feature
too. I don't seem to run into the issue with a Debian 11 (kernel 5.10)
guest, but I do run into the issue with an Ubuntu 23.10 (kernel 6.5)
guest. Seems like it got added in kernel 6.2 with 418044e1de30
("drivers/net/virtio_net.c: Added USO support.")

Best Regards,
Fiona