Re: [PATCH v5 4/6] vhost-user-blk: make inflight-migration prop mutable
"Michael S. Tsirkin" <[email protected]> Wed, 29 Jul 2026 14:50:12 -0400
| Newsgroups | dev.linux.lists.virtio-fs,org.nongnu.qemu-devel |
|---|---|
| Message-ID | <[email protected]> |
On Wed, Jul 29, 2026 at 08:18:19PM +0500, Alexandr Moshkov wrote: > > 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? Block disabling this when VM is not running. > > > > > > > > > > > > > > > >