Re: [Qemu-devel] [Qemu-stable] [PATCH v2] virtio-bus: Plug devices after features are negotiated

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 12/14/2016 08:44 AM, Cornelia Huck wrote:
>> > 14:44 < stefanha> Not sure if anyone can think of a nicer solution.
>> > 14:45 < stefanha> But we're going to have to keep lying to the guest if we want to preserve migration compatibility
>> > 14:45 < stefanha> The key change in behavior with the patch you identified is:
>> > 14:46 < stefanha> if (!virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) {
>> > 14:46 < stefanha> virtio_pci_disable_modern(proxy);
>> > 14:46 < stefanha> Previously it didn't care about vdev->host_features.  It simply allowed VERSION_1 when proxy's disable_modern boolean was false.
>> > 14:47 < mdroth> stefanha: ok, that explains why it seems to work with disable-modern=true
>> > 14:48 < stefanha> mdroth: Your Ubuntu kernel old but 14.04 LTS is definitely still around so I don't think we can ship QEMU 2.8 like this.
>> > 14:49 < stefanha> mdroth: Let's summarize it on the mailing list and see what Michael Tsirkin and Maxime Coquelin think.
>> > 14:49 < mdroth> stefanha: i suppose a potential workaround would be to tell users to set disable-modern= to match their vhost capabilities, but it's hard for them to apply that retroactively if they're looking to migrate
> Another thought: Maybe this bug only surfaced right now because older
> qemus defaulted virtio-pci to legacy?
>
> (I think modern virtio-pci with old vhost resulted in a config that was
> rejected at least by Linux guests. Because pci defaulted to legacy, we
> only had the post-plugged workaround for ccw before.)

Yes, for PCI with old vhost, modern enabled and recent kernel on guest,
we get this failure at virtio-pci probe time:

virtio_net virtio0: virtio: device uses modern interface but does not 
have VIRTIO_F_VERSION_1.

  - Maxime