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

On 7/29/26 19:48, Michael S. Tsirkin wrote:
> On Wed, Jul 29, 2026 at 07:07:11PM +0500, Alexandr Moshkov wrote:
>> 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.
> I mean it's a generic thing.
>
>> 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...
> It does not start automatically in all cases, no.
>
>> 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.
>
> Well the message is consistent. But its use does not seem to be.
>
>>>
>>>
>>>
>>>
>>>
>>>> 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.
> this:
>
> +    bool skip_drain = vhost_user_blk_inflight_needed(s) &&
> +                      runstate_check(RUN_STATE_FINISH_MIGRATE);
>
> is what I dislike.
>
> I dislike poking at migration state. It's painful enough that we
> need to worry about VM running or not.

I understand that. But I don't know how to do it another way. We can't 
just erase runstate check - it will possibly leads to missing inflight 
requests during VM stop.

And don't know how to make stop more consistent. Do you have any thoughts?

>>>
>>>
>>>
>>>