Re: [Qemu-devel] [PATCH RFC 1/1] virtio: migrate config_vector

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Wed, 13 May 2015 16:58:58 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, May 13, 2015 at 04:42:02PM +0200, Cornelia Huck wrote:
> > Currently, config_vector is managed in VirtIODevice, but migration is
> > handled by the transport; this led to one transport using config_vector
> > migrating correctly (pci), while the other forgot it (ccw).
> > 
> > Let's have the core handle migration of config_vector in a vmstate
> > subsection instead, so that we can keep it backwards compatible and
> > no additional code is needed if config_vector is not used at all. Also
> > provide a callback for virtio-pci so they can avoid introducing a
> > subsection that is not needed and still be compatible in both directions.
> > 
> > Reported-by: "Jason J. Herne" <jjherne@linux.vnet.ibm.com>
> > Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> 
> I'm inclined to say just fix ccw, don't touch core.
> queue vectors are still saved by transports, why special-case
> config_vector?

- config_vector is in the common VirtIODevice structure, not in
  transport-specific code
- new transports may make the same mistake
- AFAICS, there's no easy way to add transport-specific subsections -
  and simply adding config_vector in ccw would break compatibility

> 
> > ---
> >  hw/virtio/virtio-pci.c         | 14 ++++++++++++++
> >  hw/virtio/virtio.c             | 25 +++++++++++++++++++++++++
> >  include/hw/virtio/virtio-bus.h |  5 +++++
> >  3 files changed, 44 insertions(+)
> > 
> > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > index 867c9d1..4959b7d 100644
> > --- a/hw/virtio/virtio-pci.c
> > +++ b/hw/virtio/virtio-pci.c
> > @@ -971,6 +971,19 @@ static void virtio_pci_device_unplugged(DeviceState *d)
> >      virtio_pci_stop_ioeventfd(proxy);
> >  }
> >  
> > +static bool virtio_pci_needs_confvec(DeviceState *d)
> > +{
> > +    VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
> > +    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
> > +
> > +    /*
> > +     * We don't want the core to create an unneeded vmstate subsection
> > +     * when we already migrate the config vector ourselves.
> > +     */
> > +    return (vdev->config_vector != VIRTIO_NO_VECTOR) &&
> > +        !msix_present(&proxy->pci_dev);
> 
> As config_vector is ignored without msix,
> why is it a good idea to break migration if it's wrong?

This should be covered by the first check, no?

> 
> > +}
> > +
> >  static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp)
> >  {
> >      VirtIOPCIProxy *dev = VIRTIO_PCI(pci_dev);
> > @@ -1505,6 +1518,7 @@ static void virtio_pci_bus_class_init(ObjectClass *klass, void *data)
> >      k->device_plugged = virtio_pci_device_plugged;
> >      k->device_unplugged = virtio_pci_device_unplugged;
> >      k->query_nvectors = virtio_pci_query_nvectors;
> > +    k->needs_confvec = virtio_pci_needs_confvec;
> >  }
> >  
> >  static const TypeInfo virtio_pci_bus_info = {
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 6985e76..3af530e 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -903,6 +903,27 @@ static const VMStateDescription vmstate_virtio_device_endian = {
> >      }
> >  };
> >  
> > +static bool virtio_device_confvec_needed(void *opaque)
> > +{
> > +    VirtIODevice *vdev = opaque;
> > +    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> > +    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
> > +
> > +    return k->needs_confvec ?
> > +        k->needs_confvec(qbus->parent) :
> > +        (vdev->config_vector != VIRTIO_NO_VECTOR);
> > +}
> > +
> > +static const VMStateDescription vmstate_virtio_device_confvec = {
> > +    .name = "virtio/config_vector",
> > +    .version_id = 1,
> > +    .minimum_version_id = 1,
> > +    .fields = (VMStateField[]) {
> > +        VMSTATE_UINT16(config_vector, VirtIODevice),
> > +        VMSTATE_END_OF_LIST()
> > +    }
> > +};
> > +
> >  static const VMStateDescription vmstate_virtio = {
> >      .name = "virtio",
> >      .version_id = 1,
> > @@ -916,6 +937,10 @@ static const VMStateDescription vmstate_virtio = {
> >              .vmsd = &vmstate_virtio_device_endian,
> >              .needed = &virtio_device_endian_needed
> >          },
> > +        {
> > +            .vmsd = &vmstate_virtio_device_confvec,
> > +            .needed = &virtio_device_confvec_needed,
> > +        },
> >          { 0 }
> >      }
> >  };
> > diff --git a/include/hw/virtio/virtio-bus.h b/include/hw/virtio/virtio-bus.h
> > index a4588ca..79e6e8b 100644
> > --- a/include/hw/virtio/virtio-bus.h
> > +++ b/include/hw/virtio/virtio-bus.h
> > @@ -69,6 +69,11 @@ typedef struct VirtioBusClass {
> >       * Note that changing this will break migration for this transport.
> >       */
> >      bool has_variable_vring_alignment;
> > +    /*
> > +     * (optional) Does the bus want the core to handle config_vector
> > +     * migration? This is for backwards compatibility only.
> > +     */
> > +    bool (*needs_confvec)(DeviceState *d);
> >  } VirtioBusClass;
> >  
> >  struct VirtioBusState {
> > -- 
> > 2.1.4
>