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>

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 521ba0b41567dc1bae52e8ff56fe7e3609023e45..21c2b9dbfc97aa47662cfcc2fe66003115aaf7cf 100644 (file)
--- 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 b2a996fa8338972dd34b4beeb03490fdbde3c8e8..5e078866c42628f26fc342cce1b07cf5c41f864e 100644 (file)
--- 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 0f06e113bee2cf4e86209063a721623ae644308a..8dfc7a38c0f23d5ecc6aea679969886aa6b5793f 100644 (file)
--- 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 \