This patch introduces the mechanism in which this series will use to
detect VirtIODevice & VirtIONet state deltas, starting with the
VirtIODevice's 'status' member of a virtio-net device.

Before we send the device's state early, we save each piece of its state
in a temporary structure via virtio_net_early_pre_save. Later, once the
source VM has been paused, we compare the current values of those pieces
to what we saved earlier. If any mismatch is found, virtio-net's VMSD
(vmstate_virtio_net) is enabled and resends all state and device prep
work (as it's normally done today when live migrating a virtio-net
device).

Once all relevant delta checks are in place, a no-delta case will skip
vmstate_virtio_net and only resend updated VQ indices.

For this patch, keep a temporary always-true fallback in
virtio_net_has_delta/virtio_net_needed until follow-up patches cover the
remaining VirtIONet & VirtIODevice deltas.

Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
---
 hw/net/virtio-net.c        | 40 ++++++++++++++++++++++++++++++++++++++
 include/hw/virtio/virtio.h |  2 ++
 2 files changed, 42 insertions(+)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 5d71ad235e..2733e0130c 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -3864,6 +3864,16 @@ static bool failover_hide_primary_device(DeviceListener *listener,
     return qatomic_read(&n->failover_primary_hidden);
 }
 
+static int virtio_net_early_pre_save(void *opaque)
+{
+    VirtIONet *n = opaque;
+    VirtIODevice *vdev = VIRTIO_DEVICE(n);
+    VirtIODevMigration *vdev_mig = vdev->migration;
+
+    vdev_mig->status_early = vdev->status;
+    return 0;
+}
+
 static int virtio_net_early_pre_load(void *opaque)
 {
     VirtIONet *n = opaque;
@@ -3887,6 +3897,7 @@ static const VMStateDescription vmstate_virtio_net_early = {
     .minimum_version_id = VIRTIO_NET_VM_VERSION,
     .version_id = VIRTIO_NET_VM_VERSION,
     .early_setup = true,
+    .pre_save = virtio_net_early_pre_save,
     .pre_load = virtio_net_early_pre_load,
     .post_load = virtio_net_early_post_load,
     .fields = (const VMStateField[]) {
@@ -4231,10 +4242,39 @@ static bool dev_unplug_pending(void *opaque)
     return vdc->primary_unplug_pending(dev);
 }
 
+static bool virtio_net_has_delta(VirtIONet *n, VirtIODevice *vdev)
+{
+    VirtIODevMigration *vdev_mig = vdev->migration;
+
+    /* Has the VirtIODevice's status changed? */
+    if (vdev->status != vdev_mig->status_early) {
+        return true;
+    }
+
+    /*
+     * Always return true for now until we're able to detect all possible
+     * changes to a VirtIONet device.
+     */
+    return true;
+}
+
+static bool virtio_net_needed(void *opaque)
+{
+    VirtIONet *n = opaque;
+    VirtIODevice *vdev = VIRTIO_DEVICE(n);
+
+    if (!n->early_mig) {
+        return true;
+    }
+
+    return virtio_net_has_delta(n, vdev);
+}
+
 static const VMStateDescription vmstate_virtio_net = {
     .name = "virtio-net",
     .minimum_version_id = VIRTIO_NET_VM_VERSION,
     .version_id = VIRTIO_NET_VM_VERSION,
+    .needed = virtio_net_needed,
     .fields = (const VMStateField[]) {
         VMSTATE_VIRTIO_DEVICE,
         VMSTATE_END_OF_LIST()
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 74fed8c324..752c46ce53 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -102,9 +102,11 @@ enum virtio_device_endian {
 /**
  * struct VirtIODevMigration - Common VirtIODevice migration structure
  * @early_load: Flag to indicate an early virtio_load for the device.
+ * @status_early: Device status at the time it was sent early.
  */
 typedef struct VirtIODevMigration {
     bool early_load;
+    uint8_t status_early;
 } VirtIODevMigration;
 
 /**
-- 
2.51.0