Re: [PATCH v2 4/4] vhost-user: add skip_drain param to GET_VRING_BASE

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Mar 11, 2026 at 01:47:28PM +0500, 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
> 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.
> 
> Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
> ---
>  docs/interop/vhost-user.rst    |  6 ++----
>  hw/block/vhost-user-blk.c      | 29 +++++++++++++++++++++++------
>  hw/virtio/vhost-user.c         |  3 +--
>  hw/virtio/vhost.c              |  4 +---
>  include/hw/virtio/vhost-user.h |  1 -
>  5 files changed, 27 insertions(+), 16 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 1dc49c104e..8cc80eb0c3 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) &&
> +                      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 +579,28 @@ static struct vhost_dev *vhost_user_blk_get_vhost(VirtIODevice *vdev)
>      return &s->dev;
>  }
>  
> +static bool vhost_user_blk_pre_save(void *opaqueue, Error **errp)
> +{
> +    VHostUserBlk *s = VHOST_USER_BLK(vdev);

This won't compile. "opaqueue" is unused and the vdev variable does not
exist.

> +
> +    bool inflight_migration_enabled = vhost_user_has_protocol_feature(&s->dev,

vhost_user_has_protocol_feature() does not exist.

> +                            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 4e196fc773..c04bb53159 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -1390,12 +1390,10 @@ static int do_vhost_virtqueue_stop(struct vhost_dev *dev,
>                                     unsigned idx, bool force,
>                                     bool skip_drain)
>  {
> -    /* TODO: support skip drain */
> -    assert(!skip_drain);

The following assertion would be useful:

  if (skip_drain) {
      assert(virtio_has_feature(...protocol_features...,
              VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT));
  }

> -
>      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;
>  
>  /**
> -- 
> 2.34.1
> 

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