Re: [BUG] Migration failure between QEMU 9.2.4 → 8.2.10 due to virtio-net feature mismatch (VIRTIO_F_RING_RESET / USO features)

[Jinpu Wang <jinpu.wang@ionos.com>]

Hi Peter,
On Wed, Nov 5, 2025 at 11:17 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Wed, Nov 05, 2025 at 10:27:59AM +0100, Jinpu Wang wrote:
> > > > These are not present (or not supported) on QEMU 8.2.10, which causes
> > > > the migration state load to fail.
> > >
> > > Interesting, we've already done the compat work:
> > >
> > > GlobalProperty hw_compat_8_1[] = {
> > >     { TYPE_PCI_BRIDGE, "x-pci-express-writeable-slt-bug", "true" },
> > >     { "ramfb", "x-migrate", "off" },
> > >     { "vfio-pci-nohotplug", "x-ramfb-migrate", "off" },
> > >     { "igb", "x-pcie-flr-init", "off" },
> > >     { TYPE_VIRTIO_NET, "host_uso", "off"},
> > >     { TYPE_VIRTIO_NET, "guest_uso4", "off"},
> > >     { TYPE_VIRTIO_NET, "guest_uso6", "off"},
> > > };
> > > const size_t hw_compat_8_1_len = G_N_ELEMENTS(hw_compat_8_1);
> > Yeah, I noticed the same.
>
> AFAICT, this is a known issue..
>
> Thomas and I used to suggest we should not turn on USO* by default by
> probing kernel, but only allow user choosing it explicitly in a VM
> setup. IOW, dest qemu should stop booting at all when kernel is too old
> (when user chose the feature).
I feel this is the approach we should have picked.
>
> See:
>
> https://lore.kernel.org/all/ZqQNKZ9_OPhDq2AK@x1n/
Is there any effort to allow migration from new OS support the USO
features to old OS doesn't support it?
Any hint to make it work?
>
> Thanks,
>
> --
> Peter Xu
>
Thx for the help.