Re: [PATCH v6 5/5] vhost-user-blk: support inter-host inflight migration

[Peter Xu <peterx@redhat.com>]

On Tue, Jan 13, 2026 at 02:58:19PM +0500, Alexandr Moshkov wrote:
> During inter-host migration, waiting for disk requests to be drained
> in the vhost-user backend can incur significant downtime.
> 
> This can be avoided if QEMU migrates the inflight region in
> vhost-user-blk.
> Thus, during the qemu migration, with feature flag the vhost-user
> back-end can immediately stop vrings, so all in-flight requests will be
> migrated to another host.
> 
> Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
> ---
>  hw/block/vhost-user-blk.c          | 28 ++++++++++++++++++++++++++++
>  include/hw/virtio/vhost-user-blk.h |  1 +
>  2 files changed, 29 insertions(+)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index a8fd90480a..9093e98841 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -377,6 +377,7 @@ static int vhost_user_blk_init(DeviceState *dev, bool connect, 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,
>                           false, errp);
>      if (ret < 0) {
> @@ -656,6 +657,27 @@ static struct vhost_dev *vhost_user_blk_get_vhost(VirtIODevice *vdev)
>      return &s->dev;
>  }
>  
> +static bool vhost_user_blk_inflight_needed(void *opaque)
> +{
> +    struct VHostUserBlk *s = opaque;
> +
> +    bool inflight_migration = vhost_dev_has_feature(&s->dev,
> +                        VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
> +
> +    return inflight_migration &&
> +           !migrate_local_vhost_user_blk();

Here's the spot that should depend on migrate_local_vhost_user_blk() from
Vladimilr's RFC patch (again, likely to be renamed..).

Btw, is this check correct against "!migrate_local_vhost_user_blk()"?  I
was expecting the feature off only if local=on, so I expect it to be:

       return inflight_migration && migrate_local_vhost_user_blk();

?

> +}
> +
> +static const VMStateDescription vmstate_vhost_user_blk_inflight = {
> +    .name = "vhost-user-blk/inflight",
> +    .version_id = 1,
> +    .needed = vhost_user_blk_inflight_needed,
> +    .fields = (const VMStateField[]) {
> +        VMSTATE_VHOST_INFLIGHT_REGION(inflight, VHostUserBlk),

One other trivial nitpick while glimpsing over the patch: should we move
the macro definition from previous patch to this one, where it is used?

> +        VMSTATE_END_OF_LIST()
> +    },
> +};
> +
>  static bool vhost_user_blk_pre_incoming(void *opaque, Error **errp)
>  {
>      VHostUserBlk *s = VHOST_USER_BLK(opaque);
> @@ -678,6 +700,10 @@ static const VMStateDescription vmstate_vhost_user_blk = {
>          VMSTATE_VIRTIO_DEVICE,
>          VMSTATE_END_OF_LIST()
>      },
> +    .subsections = (const VMStateDescription * const []) {
> +        &vmstate_vhost_user_blk_inflight,
> +        NULL
> +    }
>  };
>  
>  static bool vhost_user_needed(void *opaque)
> @@ -751,6 +777,8 @@ 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),
>  };
>  
>  static void vhost_user_blk_class_init(ObjectClass *klass, const void *data)
> diff --git a/include/hw/virtio/vhost-user-blk.h b/include/hw/virtio/vhost-user-blk.h
> index b06f55fd6f..e1466e5cf6 100644
> --- a/include/hw/virtio/vhost-user-blk.h
> +++ b/include/hw/virtio/vhost-user-blk.h
> @@ -52,6 +52,7 @@ struct VHostUserBlk {
>      bool started_vu;
>  
>      bool skip_get_vring_base_on_force_shutdown;
> +    bool inflight_migration;
>  
>      bool incoming_backend;
>  };
> -- 
> 2.34.1
> 

-- 
Peter Xu