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

On 4/24/26 4:57 AM, Alexandr Moshkov wrote:
> Thanks for reply!
> 
> On 4/23/26 19:15, Raphael Norwitz wrote:
>>
>>
>> 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 was thinking about a message something like SET_VRING_ENABLE - for 
> example SET_SKIP_DRAIN_ENABLE, that would enable/disable some state 
> (skip_drain) in the backend.

I'd need to see the flow in more detail but sounds promising.

> 
>>
>>> 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;
>>>>>     /**
>>>>
>>