Re: [Qemu-devel] [PATCH v3] virtio-pci: Fix cross-version migration with older machines

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

On Wed, Dec 14, 2016 at 05:30:35PM +0100, Maxime Coquelin 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.
> 
> 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-ignore-backend-features"
> 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: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> 
> V3 fixes commit message with the new property name.
> V2 changes the naming as proposed by Michael T.and Cornelia, and
> fixes commit message.
> 
>  hw/virtio/virtio-pci.c | 5 ++++-
>  hw/virtio/virtio-pci.h | 1 +
>  include/hw/compat.h    | 4 ++++
>  3 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index 521ba0b..21c2b9d 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->ignore_backend_features &&
> +            !virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) {
>          virtio_pci_disable_modern(proxy);
>  
>          if (!legacy) {
> @@ -1852,6 +1853,8 @@ 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-ignore-backend-features", VirtIOPCIProxy,
> +                     ignore_backend_features, false),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>  
> diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
> index b2a996f..5e07886 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 ignore_backend_features;
>      OnOffAuto disable_legacy;
>      uint32_t class_code;
>      uint32_t nvectors;
> diff --git a/include/hw/compat.h b/include/hw/compat.h
> index 0f06e11..8dfc7a3 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-ignore-backend-features",\
> +        .value    = "on",\
>      },
>  
>  #define HW_COMPAT_2_6 \
> -- 
> 2.9.3