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

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

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?

> ---
>  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?

> +}
> +
>  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