virtio-pci: Fix cross-version migration with older machines
commit66d1c4c19faff83e36075c7b61053d060a8ffc50
authorMaxime Coquelin <maxime.coquelin@redhat.com>
Wed, 14 Dec 2016 16:30:35 +0000 (14 17:30 +0100)
committerStefan Hajnoczi <stefanha@redhat.com>
Thu, 15 Dec 2016 07:35:19 +0000 (15 07:35 +0000)
treed85a1575a0856dcc9e1cc8ba1f0446afd647b754
parentd70678a5388fb6340d018f51b863ec9712b905da
virtio-pci: Fix cross-version migration with older machines

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>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Message-id: 20161214163035.3297-1-maxime.coquelin@redhat.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>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
hw/virtio/virtio-pci.c
hw/virtio/virtio-pci.h
include/hw/compat.h