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

On 7/29/26 17:13, Michael S. Tsirkin wrote:
> On Wed, Jul 29, 2026 at 03:56:54PM +0500, Alexandr Moshkov wrote:
>> 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
> restarting you mean.
Yes, sorry.
>
>> (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).
> so again it's not the 1st feature we have like this. we tie migration
> to the machine type and same set of command line flags
> specifically to keep things manageable.
>
> really cross version migration is a pain as it is.

I understand that, thanks! Maybe we should focus on the fact that this 
series allows to turn inflight_migration on and off (without cross 
version migration) ?

Firstly, I wanted to implement this through migration capabilities, but 
in one of the threads,
it was agreed that a separate capability only for vhost-user-blk is not 
a good idea.
Therefore, I implemented it through a parameter that can be changed 
using qom-set.

>
>>                           ---
>>                            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.
>
> Sorry, to be more clear. I mean migration succeeded but destination failed to
> start, so now vm is stopped and we are now trying migrating to a different destination.

Is this a real case? I was thinking in case of any destination errors 
(migration process or starting), source vm has to work...

I guess in that case, inflight requests can only be executed on 
different destination or in current destination if vm be able to start.

> IIUC currently on VM stop we simply send GET_BASE and this stops
> backend, and depending on features things remain in the inflight buffer.
> Correct me if  I am wrong.
Yes backend has to disconnect from QEMU.. So it not be changing guest 
memory.
>
> I think that we do not want a slow flush on vm stop if we
> can avoid it, and we also do not want backend to keep changing
> guest memory when VM is stopped. No?
Yes, it is
>>
>>          skip_drain true only if inflight_migration is on and runstate is
>>          FINISH_MIGRATION.
> FINISH_MIGRATE? actually i do not see where it affects it.

Sorry, I didn't understand) Who affects who?

>
>>      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 am not 100% sure it's implementable as described, but I guess
> you could block changing the property if VM and thus the backend
> is not connected and running.
>
>
> Which is even more complexity but at least it is consistent.

Oh, I understand what you thinking about.

I agreed, but why implementation (in this series) is not consistent? I 
was thinking that current (in current qemu version) impl is not, because 
GET_VRING_BASE has two behaviors, depending on the presence of the 
protocol feature (drain or not drain requests).

Now, I think it would be more clear: one message for drain 
GET_VRING_BASE (as it was before), one for not drain 
GET_VRING_BASE_SKIP_DRAIN.

>
>
>
>
>
>
>> I think backend must perform
>> drain in all cases except live migration with all the necessary checks
>> (protocol features, special message GET_VRING_BASE_SKIP_DRAIN).
>
> This is not what is going on now, right? Whether it drains depends
> on protocol features not on VM state and I think we should
> keep it like this.

Well yes, now in current qemu version drain or not drain depends on 
protocol feature only, that try to be negotiated only if 
inflight_migration is set on.

In patch: in case live migration (if protocol feature and  device param) 
sent message GET_VRING_BASE_SKIP_DRAIN to skip drain, in other cases 
sent other message - GET_VRING_BASE with draining all requests.
So drain or not drain depends on message that QEMU send to backend.

>
>
>>
>>              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
>>
>>
>
> i mean it migrated with on. vm was stopped then it flipped to off.
After stop vm will send GET_VRING_BASE to backend, that lead to drain 
all requests. Non of the inflight requests will be missed here, If 
that's what you mean.
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