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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 29, 2026 at 03:35:31PM +0500, Alexandr Moshkov wrote:
> 
> On 7/29/26 15:14, Michael S. Tsirkin wrote:
> > 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 <rnorwitz@nvidia.com>
> > >          Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
> > > 
> > > 
> > > 
> > >      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.
> 
> In case of VM stop, backend wait to drain all requests. Ability to perform
> drain or not available only during migration by skip_drain variable in
> vhost_user_blk_stop().

What if it was migration but it failed?

> skip_drain true only if inflight_migration is on and runstate is
> FINISH_MIGRATION.

This btw I don't much like, a stopped VM would preferably
behave the same whatever the reason to stop.

> 
> > i turn inflight off.
> > now it looks like it will happily migrate?
> 
> So in this case, when vm migrated with inflight off, it will leads to using
> GET_VRING_BASE message, that wait all requests to be drained.
> 
> > 
> > >          --
> > >          2.34.1
> > >