Re: [Qemu-devel] [PATCH] virtio-pci: Fix cross-version migration with older machines

[Maxime Coquelin]

On 12/14/2016 04:15 PM, Michael S. Tsirkin wrote:
> On Wed, Dec 14, 2016 at 09:08:52AM -0600, Michael Roth wrote:
> > Quoting Maxime Coquelin (2016-12-14 06:52:37)
> > > This patch fixes a cross-version migration regression introduced
> > > by commit d1b4259f ("virtio-bus: Plug devices after features are
> > > negotiated").
> > >
> > > The problem is encountered when host's vhost backend does not support
> > > VIRTIO_F_VERSION_1, and migration is initiated from a v2.7 or prior
> > > machine with virtio-pci modern capabilities enabled to a v2.8 machine.
> > >
> > > In this case, modern capabilities get exposed to the guest by the source,
> > > whereas the target will detect version 1 is not supported so will only
> > > expose legacy capabilities.
> > >
> > > The problem is fixed by introducing a new "x-modern-broken" property,
> > > which is set in v2.7 and prior compatibility modes. Doing this, v2.7
> > > machine keeps its broken behaviour (enabling modern while version is
> > > not supported), and newer machines will behave correctly.
> > >
> > > Reported-by: Michael Roth <address@hidden>
> > > Suggested-by: Stefan Hajnoczi <address@hidden>
> > > Cc: Michael S. Tsirkin <address@hidden>
> > > Cc: Cornelia Huck <address@hidden>
> > > Cc: Marcel Apfelbaum <address@hidden>
> > > Cc: Dr. David Alan Gilbert <address@hidden>
> > > Signed-off-by: Maxime Coquelin <address@hidden>
> >
> > Tested-by: Michael Roth <address@hidden>
> >
> > I can confirm this fixes the original issue I reported. I also did a
> > number of sanity runs with 2.7/2.8/pc-i440fx-2.{6,7,8} using various
> > combinations of disable-modern=true/false on hosts with/without virtio-1,
> > and some tests with pseries machines as well, and everything seems to
> > work.
> >
> > Thanks for the quick fix!
>
> FYI what I think does not work is a recent kernel on 2.7
> machine type and host without virtio 1.
> But this is not new.

I confirm, and as we discussed off-list, I will propose a kernel patch
to improve the situation, even if it will not fix current guests using
recent kernels.


> > > ---
> > >
> > > I'm not sure about the property name, let me know if you have better ideas.
> > > I didn't tested migration yet, but I wanted to share the patch while I test it.
> > > I tested booting v2.8 and v2.7 machines with !VERSION_1 and get expected result:
> > >  - v2.8: Virtio-pci probe succeed
> > >  - v2.7: Virtio-pci probe fails
> > >
> > > Thanks,
> > > Maxime
> > >
> > >  hw/virtio/virtio-pci.c | 4 +++-
> > >  hw/virtio/virtio-pci.h | 1 +
> > >  include/hw/compat.h    | 4 ++++
> > >  3 files changed, 8 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > > index 521ba0b..93f6b54 100644
> > > --- a/hw/virtio/virtio-pci.c
> > > +++ b/hw/virtio/virtio-pci.c
> > > @@ -1580,7 +1580,8 @@ static void virtio_pci_device_plugged(DeviceState *d, 
> > > Error **errp)
> > >       * Virtio capabilities present without
> > >       * VIRTIO_F_VERSION_1 confuses guests
> > >       */
> > > -    if (!virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) {
> > > +    if (!proxy->modern_broken &&
> > > +            !virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) {
> > >          virtio_pci_disable_modern(proxy);
> > >
> > >          if (!legacy) {
> > > @@ -1852,6 +1853,7 @@ static Property virtio_pci_properties[] = {
> > >                      VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT, false),
> > >      DEFINE_PROP_BIT("page-per-vq", VirtIOPCIProxy, flags,
> > >                      VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT, false),
> > > +    DEFINE_PROP_BOOL("x-modern-broken", VirtIOPCIProxy, modern_broken, false),
> > >      DEFINE_PROP_END_OF_LIST(),
> > >  };
> > >
> > > diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
> > > index b2a996f..1dca223 100644
> > > --- a/hw/virtio/virtio-pci.h
> > > +++ b/hw/virtio/virtio-pci.h
> > > @@ -153,6 +153,7 @@ struct VirtIOPCIProxy {
> > >      int config_cap;
> > >      uint32_t flags;
> > >      bool disable_modern;
> > > +    bool modern_broken;
> > >      OnOffAuto disable_legacy;
> > >      uint32_t class_code;
> > >      uint32_t nvectors;
> > > diff --git a/include/hw/compat.h b/include/hw/compat.h
> > > index 0f06e11..fe11723 100644
> > > --- a/include/hw/compat.h
> > > +++ b/include/hw/compat.h
> > > @@ -18,6 +18,10 @@
> > >          .driver   = "intel-iommu",\
> > >          .property = "x-buggy-eim",\
> > >          .value    = "true",\
> > > +    },{\
> > > +        .driver   = "virtio-pci",\
> > > +        .property = "x-modern-broken",\
> > > +        .value    = "on",\
> > >      },
> > >
> > >  #define HW_COMPAT_2_6 \
> > > --
> > > 2.9.3