Re: [PATCH v5 4/6] vhost-user-blk: make inflight-migration prop mutable
Alexandr Moshkov <[email protected]> Thu, 30 Jul 2026 14:01:02 +0500
| Newsgroups | dev.linux.lists.virtio-fs,org.nongnu.qemu-devel |
|---|---|
| Message-ID | <[email protected]> |
On 7/29/26 23:50, Michael S. Tsirkin wrote: > 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. Alright, thanks. Now I understand) > >>>>> >>>>> >>>>>