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

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 <[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.

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().
skip_drain true only if inflight_migration is on and runstate is 
FINISH_MIGRATION.

> 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
>>