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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 14, 2016 at 02:12:10PM +0100, Cornelia Huck wrote:
> On Wed, 14 Dec 2016 13:52:37 +0100
> Maxime Coquelin <maxime.coquelin@redhat.com> wrote:
> 
> > 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.
> 
> Wait, machine versions or qemu versions?
> 
> > 
> > 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 <mdroth@linux.vnet.ibm.com>
> > Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
> > Cc: Marcel Apfelbaum <marcel@redhat.com>
> > Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > ---
> > 
> > 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 &&
> 
> What you are de facto doing here is ignoring the features supported by
> the backend. Call this ->ignore_backend_features or so?
> 
> > +            !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),
> 
> What about "x-ignore-backend-features"?

It's just the capability that ignores that backend is legacy.

x-cap-ignore-backend-legacy

?

> >      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",\
> 
> This unfortunately has the same problem wrt -global propagation as the
> other virtio-pci compat props (unexpeced overrides). There's basically
> zero chance, however, that somebody will want to set this property
> manually (unline disable-legacy/disable-modern), and there's a patch
> targeting 2.8.1, so I think we can live with it. (Alternatively, you
> would need to set this property on all possible virtio-pci devices.)

YEs.

> >      },
> > 
> >  #define HW_COMPAT_2_6 \