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

["Michael S. Tsirkin" <mst@redhat.com>]

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 <rnorwitz@nvidia.com>
> > >                           Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
> > > 
> > > 
> > > 
> > >                       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.

> > 
> > 
> > 
> > 
> >