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.
> >
> >
> >
> >
> >
