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

On 7/29/26 15:48, Michael S. Tsirkin wrote:
> 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<[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().
> What if it was migration but it failed?
If migration failed, source vm just keep using inflight region, backend 
continue to execute inflight requests that was tried to migrate.
>
>> 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.
Well, I don't see any other way to implement this. I think backend must 
perform drain in all cases except live migration with allthe 
necessarychecks (protocol features, special message 
GET_VRING_BASE_SKIP_DRAIN).
>
>>> 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
>>>>