Re: [PATCH v5 4/6] vhost-user-blk: make inflight-migration prop mutable

On Wed, Jul 29, 2026 at 03:09:57PM +0500, Alexandr Moshkov wrote:
> 
> 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.  


I apologise i reverted the logic in the question.


I start vm and inflight is on.
I stop vm get base does not drain.
i turn inflight off.
now it looks like it will happily migrate?

>         --
>         2.34.1
>