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

[Fabiano Rosas <farosas@suse.de>]

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

> 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.")

Ah ok, so this is more complex, the tests wouldn't have caught it even
with the device options addition. A test for this will have to come at a
second moment once we figure out how to deal with guest-code dependent
issues.