On Wed, 14 Dec 2016 08:41:55 +0000 Stefan Hajnoczi <[email protected]> wrote:
> On Wed, Dec 14, 2016 at 8:28 AM, Maxime Coquelin > <[email protected]> wrote: > > > > > > 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. > > Is this error a regression in QEMU 2.8? I think it pokes up because modern virtio-pci is now by default on. It was broken before if the user wanted a modern virtio-pci device explicitly. (ccw defaulted to virtio 1.0 much earlier, so we had the post-plugged solution that this patch replaced and which is basically the same for ccw.) > > It's better to ship with an existing issue still open than with a new > regression. We must not break existing users' setups. > > A solution for the next QEMU version is to use a flag in the machine > type version telling virtio whether or not allow devices (e.g. > vhost-net) to influence the host feature bits. Old machine types will > say no, new machine types will say yes. > > In the meantime I would revert your patch for QEMU 2.8. > > Maxime, Cornelia, Michael: Do you agree? > > Stefan Reverting the patch should be fine for ccw. What about the virtio-pci with old vhost mess, though? Defaulting to modern would mean that users get unusable devices in that setup.
