virtio-balloon-pci: Allow setting nvectors, so we can use MSI-X

Most virtio-pci devices allow MSI-X. Add it to virtio-balloon-pci, but
only enable it in new machine types, so we don't break migration of
existing machine types between different qemu versions.

This copies what was done for virtio-rng-pci in:
9ea02e8f1306 ("virtio-rng-pci: Allow setting nvectors, so we can use MSI-X")
bad9c5a5166f ("virtio-rng-pci: fix migration compat for vectors")
62bdb8871512 ("virtio-rng-pci: fix transitional migration compat for vectors")

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Message-ID: <20250115161425.246348-1-arbab@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/core/machine.c

@@ -38,6 +38,9 @@
 
 GlobalProperty hw_compat_9_2[] = {
     {"arm-cpu", "backcompat-pauth-default-use-qarma5", "true"},
+    { "virtio-balloon-pci", "vectors", "0" },
+    { "virtio-balloon-pci-transitional", "vectors", "0" },
+    { "virtio-balloon-pci-non-transitional", "vectors", "0" },
 };
 const size_t hw_compat_9_2_len = G_N_ELEMENTS(hw_compat_9_2);
 

hw/virtio/virtio-balloon-pci.c

@@ -35,11 +35,22 @@ struct VirtIOBalloonPCI {
     VirtIOBalloon vdev;
 };
 
+static const Property virtio_balloon_properties[] = {
+    DEFINE_PROP_BIT("ioeventfd", VirtIOPCIProxy, flags,
+                    VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT, true),
+    DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors,
+                       DEV_NVECTORS_UNSPECIFIED),
+};
+
 static void virtio_balloon_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
 {
     VirtIOBalloonPCI *dev = VIRTIO_BALLOON_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
 
+    if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
+        vpci_dev->nvectors = 2;
+    }
+
     vpci_dev->class_code = PCI_CLASS_OTHERS;
     qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
 }
@@ -55,6 +66,7 @@ static void virtio_balloon_pci_class_init(ObjectClass *klass, void *data)
     pcidev_k->device_id = PCI_DEVICE_ID_VIRTIO_BALLOON;
     pcidev_k->revision = VIRTIO_PCI_ABI_VERSION;
     pcidev_k->class_id = PCI_CLASS_OTHERS;
+    device_class_set_props(dc, virtio_balloon_properties);
 }
 
 static void virtio_balloon_pci_instance_init(Object *obj)