On 7/29/26 14:32, Michael S. Tsirkin wrote:
On Tue, Jul 28, 2026 at 03:08:39PM +0500, Alexandr Moshkov wrote:
When migrating from a QEMU version that supports inflight-migration to
an older one that does not, there is no way to disable the feature at
runtime — the VM must be stopped and reconfigured. This is impractical
in production environments.
Make the inflight-migration property mutable after device realization
so it can be toggled via qom-set without restarting the VM.
Acked-by: Raphael Norwitz <[email protected]>
Signed-off-by: Alexandr Moshkov <[email protected]>
I think I am beginning to understand.
You are running qemu with inflight-migration on and want to migrate
to qemu without inflight-migration at all.
Since it is guest transparent you could retrofit it like this.
My question is why is it worth it, we do not normally support
migrating between qemu versions with different command lines.
I think you right. Maybe I've been focusing too much on the ability to migrate
between versions of qemu with and without inflight migration.
This series allows to turn off and on the inflight-migration feature at runtime
without recreating the VM (it was the only way to turn off the feature, since
the protocol feature could no longer be turned off, after initialization with
the backend). This is more important feature, and it leads to the fact that
this feature allows to migrate between different versions of qemu (with and
without inflight-migration support).
---
hw/block/vhost-user-blk.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index e3b873af7c..650c004bde 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -619,6 +619,8 @@ static const VMStateDescription
vmstate_vhost_user_blk = {
}
};
+static PropertyInfo vhost_user_blk_inflight_migration_prop;
+
static const Property vhost_user_blk_properties[] = {
DEFINE_PROP_CHR("chardev", VHostUserBlk, chardev),
DEFINE_PROP_UINT16("num-queues", VHostUserBlk, num_queues,
@@ -632,8 +634,9 @@ static const Property vhost_user_blk_properties[]
= {
VIRTIO_BLK_F_WRITE_ZEROES, true),
DEFINE_PROP_BOOL("skip-get-vring-base-on-force-shutdown",
VHostUserBlk,
skip_get_vring_base_on_force_shutdown, false),
- DEFINE_PROP_BOOL("inflight-migration", VHostUserBlk,
- inflight_migration, false),
+ DEFINE_PROP("inflight-migration", VHostUserBlk,
inflight_migration,
+ vhost_user_blk_inflight_migration_prop, bool,
+ .set_default = true, .defval.u = false),
};
static void vhost_user_blk_class_init(ObjectClass *klass, const void
*data)
@@ -665,6 +668,9 @@ static const TypeInfo vhost_user_blk_info = {
static void virtio_register_types(void)
{
+ vhost_user_blk_inflight_migration_prop = qdev_prop_bool;
+ vhost_user_blk_inflight_migration_prop.realized_set_allowed =
true;
+
type_register_static(&vhost_user_blk_info);
}
So then, for example, let us say I paused the VM, then set the flag,
now inflight is on but GET_BASE did not drain it?
If I understood the question correctly, this is valid behavior. Before
migration QEMU check protocol features to understand does the backend support
inflight migration. If it does, after that QEMU migrate inflight buffer to
other VM. If it's not, return error before migration started.
