[PATCH 2/2] virtio: failover: define the default device to use in case of error

[Laurent Vivier]

If the guest driver doesn't support the STANDBY feature, by default
we keep the virtio-net device and don't hotplug the VFIO device,
but in some cases, user can prefer to use the VFIO device rather
than the virtio-net one. We can't unplug the virtio-net device
(because on migration it is expected on the destination side)
but we can force the guest driver to be disabled. Then, we can
hotplug the VFIO device that will be unplugged before the migration
like in the normal failover migration but without the failover device.

This patch adds a new property to virtio-net device: "failover-default".

By default, "failover-default" is set to true and thus the default NIC
to use if the failover cannot be enabled is the virtio-net device
(this is what is done until now with the virtio-net failover).

If "failover-default" is set to false, in case of error, the virtio-net
device is not the default anymore and the failover primary device
is used instead.

If the STANDBY feature is supported by guest and host, the virtio-net
failover acts as usual.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 include/hw/virtio/virtio-net.h |  1 +
 hw/net/virtio-net.c            | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
index 824a69c23f06..ab77930a327e 100644
--- a/include/hw/virtio/virtio-net.h
+++ b/include/hw/virtio/virtio-net.h
@@ -208,6 +208,7 @@ struct VirtIONet {
     /* primary failover device is hidden*/
     bool failover_primary_hidden;
     bool failover;
+    bool failover_default;
     DeviceListener primary_listener;
     Notifier migration_state;
     VirtioNetRssData rss_data;
diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 16d20cdee52a..6fe0a09a263b 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -891,6 +891,39 @@ static void virtio_net_set_features(VirtIODevice *vdev, 
uint64_t features)
     Error *err = NULL;
     int i;
 
+    /*
+     * If the guest driver doesn't support the STANDBY feature, by default
+     * we keep the virtio-net device and don't hotplug the VFIO device,
+     * but in some cases, user can prefer to use the VFIO device rather
+     * than the virtio-net one. We can't unplug the virtio-net device
+     * (because on migration it is expected on the destination side)
+     * but we can force the guest driver to be disabled. Then, we can
+     * hotplug the VFIO device that will be unplugged before the migration
+     * like in the normal failover migration but without the failover device.
+     */
+    if (n->failover && !n->failover_default) {
+        if (!virtio_has_feature(features, VIRTIO_NET_F_STANDBY)) {
+            /* disable the first queue to disable the driver */
+            virtio_queue_disable(vdev, 0);
+            /*
+             * as the virtio-net driver is disable we can plug back the
+             * failover primary device
+             */
+            qatomic_set(&n->failover_primary_hidden, false);
+            failover_add_primary(n, &err);
+            if (err) {
+                warn_report_err(err);
+            }
+            return;
+        } else {
+          /*
+           * if the driver renegotiates features, we need to re-enable
+           * the queue
+           */
+          virtio_queue_enable(vdev, 0);
+        }
+    }
+
     if (n->mtu_bypass_backend &&
             !virtio_has_feature(vdev->backend_features, VIRTIO_NET_F_MTU)) {
         features &= ~(1ULL << VIRTIO_NET_F_MTU);
@@ -3625,6 +3658,7 @@ static Property virtio_net_properties[] = {
     DEFINE_PROP_INT32("speed", VirtIONet, net_conf.speed, SPEED_UNKNOWN),
     DEFINE_PROP_STRING("duplex", VirtIONet, net_conf.duplex_str),
     DEFINE_PROP_BOOL("failover", VirtIONet, failover, false),
+    DEFINE_PROP_BOOL("failover-default", VirtIONet, failover_default, true),
     DEFINE_PROP_END_OF_LIST(),
 };
 
-- 
2.31.1