Re: [PATCH v3 5/5] vhost-user: add skip_drain param to GET_VRING_BASE

On 4/16/26 5:26 AM, Alexandr Moshkov wrote:
> Greetings! Thanks for reply!
> 
> 
> On 4/15/26 20:21, Raphael Norwitz wrote:
>> I’m not sure I like using the num field in vhost_vring_state to set 
>> magic values which affect protocol behavior for GET_VRING_BASE. It 
>> feels like a hack to me. I would think the proper solution if we want 
>> to support migration from new to old would be to have new use a 
>> different new message entirely. Can we do that?
> 
> I think we can, but I thought at first that this will be almost a 
> complete copy of GET_VRING_BASE message, with the exception of waiting 
> for drain of requests, so I choose to expand existing message.
> 

I think a new message would be cleaner. Anyone else have thoughts?

> If this is not an appropriate approach, is it better to make a new 
> message like GET_VRING_BASE or a separate message used together with 
> default GET_VRING_BASE message (for example, message for setting some 
> kind of status on the server)? What should I name this new message?
> 

Maybe GET_VRING_BASE_SKIP_DRAIN? How would a separate message used 
together with default GET_VRING_BASE message work?

> I also fix other comments, thanks!
> 
>>
>> On 3/30/26 11:52 AM, Alexandr Moshkov wrote:
>>> In case of migration of QEMU from the new version (where the
>>> inllight-migration parameter is present), to the old one (where it is
>>
>> nit: spelling inllight-migration
>>
>>> absent) there is no way to disable this feature on the backend during
>>> runtime.
>>>
>>> This commit slightly changes the semantics of the protocol feature
>>> VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT. Enabling this feature
>>> adds a new parameter for GET_VRING_BASE, which allows to control the
>>> drain in-flight requests on the backend.
>>> Thus, QEMU will be able to turn this feature on GET_VRING_BASE off and
>>> on anytime.
>>>
>>> In vhost-user-blk use inflight_migration param to enable skip_drain to
>>> suspend in-flight I/O requests, and then migrate them throught inflight
>>> subsection.
>>>
>>> Also now QEMU will always try to setup
>>> VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT protocol featrue with
>>> backend. This will allow to use skip_drain parameter on GET_VRING_BASE
>>> message.
>>>
>>> Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
>>> Signed-off-by: Alexandr Moshkov <[email protected]>
>>> ---
>>>   docs/interop/vhost-user.rst    |  6 ++----
>>>   hw/block/vhost-user-blk.c      | 30 ++++++++++++++++++++++++------
>>>   hw/virtio/vhost-user.c         |  3 +--
>>>   hw/virtio/vhost.c              |  1 +
>>>   include/hw/virtio/vhost-user.h |  1 -
>>>   5 files changed, 28 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
>>> index bfa75ff9a3..63efc87264 100644
>>> --- a/docs/interop/vhost-user.rst
>>> +++ b/docs/interop/vhost-user.rst
>>> @@ -1255,14 +1255,12 @@ Front-end message types
>>>     *suspended*, see :ref:`Suspended device state
>>>     <suspended_device_state>`.
>>>   -  The request payload's *num* field is currently reserved and must be
>>> -  set to 0.
>>> -
>>>     By default, the back-end must complete all inflight I/O requests 
>>> for the
>>>     specified vring before stopping it.
>>>       If the ``VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT`` protocol
>>> -  feature has been negotiated, the back-end may suspend in-flight I/O
>>> +  feature has been negotiated, using request payload's *num* field,
>>> +  when *num* is set to 1, QEMU can tell the back-end to suspend in- 
>>> flight I/O
>>>     requests and record them as described in :ref:`Inflight I/O tracking
>>>     <inflight_io_tracking>` instead of completing them before 
>>> stopping the vring.
>>>     How to suspend an in-flight request depends on the implementation 
>>> of the back-end
>>> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
>>> index a3ecd83f54..522e9d34b5 100644
>>> --- a/hw/block/vhost-user-blk.c
>>> +++ b/hw/block/vhost-user-blk.c
>>> @@ -134,10 +134,7 @@ static bool vhost_user_blk_inflight_needed(void 
>>> *opaque)
>>>   {
>>>       struct VHostUserBlk *s = opaque;
>>>   -    bool inflight_migration = virtio_has_feature(s- 
>>> >dev.protocol_features,
>>> - VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
>>> -
>>> -    return inflight_migration;
>>> +    return s->inflight_migration;
>>>   }
>>>     @@ -232,11 +229,14 @@ static int vhost_user_blk_stop(VirtIODevice 
>>> *vdev)
>>>           return 0;
>>>       }
>>>   +    bool skip_drain = vhost_user_blk_inflight_needed(s) &&
>>
>> nit: declarations on top
>>
>>> + runstate_check(RUN_STATE_FINISH_MIGRATE);
>>> +
>>>       force_stop = s->skip_get_vring_base_on_force_shutdown &&
>>>                    qemu_force_shutdown_requested();
>>>         ret = force_stop ? vhost_dev_force_stop(&s->dev, vdev, true) :
>>> -                       vhost_dev_stop(&s->dev, vdev, true, false);
>>> +                       vhost_dev_stop(&s->dev, vdev, true, skip_drain);
>>>         if (k->set_guest_notifiers(qbus->parent, s->dev.nvqs, false) 
>>> < 0) {
>>>           error_report("vhost guest notifier cleanup failed: %d", ret);
>>> @@ -364,7 +364,6 @@ static int vhost_user_blk_connect(DeviceState 
>>> *dev, Error **errp)
>>>       vhost_dev_set_config_notifier(&s->dev, &blk_ops);
>>>         s->vhost_user.supports_config = true;
>>> -    s->vhost_user.supports_inflight_migration = s->inflight_migration;
>>>       ret = vhost_dev_init(&s->dev, &s->vhost_user, 
>>> VHOST_BACKEND_TYPE_USER, 0,
>>>                            errp);
>>>       if (ret < 0) {
>>> @@ -580,10 +579,29 @@ static struct vhost_dev 
>>> *vhost_user_blk_get_vhost(VirtIODevice *vdev)
>>>       return &s->dev;
>>>   }
>>>   +static bool vhost_user_blk_pre_save(void *opaque, Error **errp)
>>> +{
>>> +    VHostUserBlk *s = VHOST_USER_BLK(opaque);
>>> +
>>> +    bool inflight_migration_enabled = virtio_has_feature(
>>> +        s->dev.protocol_features,
>>> +        VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
>>> +    if (vhost_user_blk_inflight_needed(s) && ! 
>>> inflight_migration_enabled) {
>>> +        error_setg(errp, "can't migrate vhost-user-blk device: "
>>> +                         "backend doesn't support "
>>> + "VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT "
>>> +                         "protocol feature");
>>> +        return false;
>>> +    }
>>> +
>>> +    return true;
>>> +}
>>> +
>>>   static const VMStateDescription vmstate_vhost_user_blk_inflight = {
>>>       .name = "vhost-user-blk/inflight",
>>>       .version_id = 1,
>>>       .needed = vhost_user_blk_inflight_needed,
>>> +    .pre_save_errp = vhost_user_blk_pre_save,
>>>       .fields = (const VMStateField[]) {
>>>           VMSTATE_VHOST_INFLIGHT_REGION(inflight, VHostUserBlk),
>>>           VMSTATE_END_OF_LIST()
>>> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
>>> index bb8f8eab77..ed95ec7523 100644
>>> --- a/hw/virtio/vhost-user.c
>>> +++ b/hw/virtio/vhost-user.c
>>> @@ -2225,8 +2225,7 @@ static int vhost_user_backend_init(struct 
>>> vhost_dev *dev, void *opaque,
>>>               }
>>>           }
>>>   -        if (!u->user->supports_inflight_migration ||
>>> -            !virtio_has_feature(protocol_features,
>>> +        if (!virtio_has_feature(protocol_features,
>>> VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD)) {
>>>               protocol_features &= ~(1ULL <<
>>> VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>>> index ac4d6fca73..54e4ce2660 100644
>>> --- a/hw/virtio/vhost.c
>>> +++ b/hw/virtio/vhost.c
>>> @@ -1398,6 +1398,7 @@ static int do_vhost_virtqueue_stop(struct 
>>> vhost_dev *dev,
>>>       int vhost_vq_index = dev->vhost_ops->vhost_get_vq_index(dev, idx);
>>>       struct vhost_vring_state state = {
>>>           .index = vhost_vq_index,
>>> +        .num = skip_drain,
>>>       };
>>>       int r = 0;
>>>   diff --git a/include/hw/virtio/vhost-user.h b/include/hw/virtio/ 
>>> vhost-user.h
>>> index 53fe996686..c95bad5ddc 100644
>>> --- a/include/hw/virtio/vhost-user.h
>>> +++ b/include/hw/virtio/vhost-user.h
>>> @@ -69,7 +69,6 @@ typedef struct VhostUserState {
>>>       GPtrArray *notifiers;
>>>       int memory_slots;
>>>       bool supports_config;
>>> -    bool supports_inflight_migration;
>>>   } VhostUserState;
>>>     /**
>>