From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>

DEFINE_PROP_ON_OFF_AUTO() property isn't runtime-mutable so using it
would mean that the source VM would need to decide upfront at startup
time whether it wants to do a multifd device state transfer at some
point.

Source VM can run for a long time before being migrated so it is
desirable to have a fallback mechanism to the old way of transferring
VFIO device state if it turns to be necessary.

This brings this property to the same mutability level as ordinary
migration parameters, which too can be adjusted at the run time.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/f2f2d66bda477da3e6cb8c0311006cff36e8651d.1741124640.git.maciej.szmigiero@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
---
 hw/vfio/migration-multifd.c |  4 ++++
 hw/vfio/pci.c               | 20 +++++++++++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/hw/vfio/migration-multifd.c b/hw/vfio/migration-multifd.c
index aacddc503bb8ba6c40e38c97ce9e2b3ca9430e12..233724710b377afd40aabe5ef5b846f654f9865d 100644
--- a/hw/vfio/migration-multifd.c
+++ b/hw/vfio/migration-multifd.c
@@ -485,6 +485,10 @@ bool vfio_multifd_setup(VFIODevice *vbasedev, bool alloc_multifd, Error **errp)
 {
     VFIOMigration *migration = vbasedev->migration;
 
+    /*
+     * Make a copy of this setting at the start in case it is changed
+     * mid-migration.
+     */
     if (vbasedev->migration_multifd_transfer == ON_OFF_AUTO_AUTO) {
         migration->multifd_transfer = vfio_multifd_transfer_supported();
     } else {
diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 1bbf15cea32657ef2a9ee37b8e1dee392d5dbcb7..fdbc15885d442ee20343e6b7127d54a70b1688ce 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -3357,6 +3357,8 @@ static void vfio_instance_init(Object *obj)
     pci_dev->cap_present |= QEMU_PCI_CAP_EXPRESS;
 }
 
+static PropertyInfo vfio_pci_migration_multifd_transfer_prop;
+
 static const Property vfio_pci_dev_properties[] = {
     DEFINE_PROP_PCI_HOST_DEVADDR("host", VFIOPCIDevice, host),
     DEFINE_PROP_UUID_NODEFAULT("vf-token", VFIOPCIDevice, vf_token),
@@ -3381,9 +3383,10 @@ static const Property vfio_pci_dev_properties[] = {
                     VFIO_FEATURE_ENABLE_IGD_OPREGION_BIT, false),
     DEFINE_PROP_ON_OFF_AUTO("enable-migration", VFIOPCIDevice,
                             vbasedev.enable_migration, ON_OFF_AUTO_AUTO),
-    DEFINE_PROP_ON_OFF_AUTO("x-migration-multifd-transfer", VFIOPCIDevice,
-                            vbasedev.migration_multifd_transfer,
-                            ON_OFF_AUTO_AUTO),
+    DEFINE_PROP("x-migration-multifd-transfer", VFIOPCIDevice,
+                vbasedev.migration_multifd_transfer,
+                vfio_pci_migration_multifd_transfer_prop, OnOffAuto,
+                .set_default = true, .defval.i = ON_OFF_AUTO_AUTO),
     DEFINE_PROP_BOOL("migration-events", VFIOPCIDevice,
                      vbasedev.migration_events, false),
     DEFINE_PROP_BOOL("x-no-mmap", VFIOPCIDevice, vbasedev.no_mmap, false),
@@ -3608,6 +3611,17 @@ static const TypeInfo vfio_pci_nohotplug_dev_info = {
 
 static void register_vfio_pci_dev_type(void)
 {
+    /*
+     * Ordinary ON_OFF_AUTO property isn't runtime-mutable, but source VM can
+     * run for a long time before being migrated so it is desirable to have a
+     * fallback mechanism to the old way of transferring VFIO device state if
+     * it turns to be necessary.
+     * The following makes this type of property have the same mutability level
+     * as ordinary migration parameters.
+     */
+    vfio_pci_migration_multifd_transfer_prop = qdev_prop_on_off_auto;
+    vfio_pci_migration_multifd_transfer_prop.realized_set_allowed = true;
+
     type_register_static(&vfio_pci_dev_info);
     type_register_static(&vfio_pci_nohotplug_dev_info);
 }
-- 
2.48.1