Re: [PATCH v7 5/5] vhost-user-blk: support inter-host inflight migration

Sun, 18 Jan 2026 22:44:57 -0800 


On 1/17/26 00:50, Raphael Norwitz wrote:

Apologies if I've missed something obvious here - what actually skips
the GET_VRING_BASE operation here? I was expecting something like:

--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -221,8 +221,9 @@ static int vhost_user_blk_stop(VirtIODevice *vdev)
          return 0;
      }

-    force_stop = s->skip_get_vring_base_on_force_shutdown &&
-                 qemu_force_shutdown_requested();
+    force_stop = (s->skip_get_vring_base_on_force_shutdown &&
+                 qemu_force_shutdown_requested()) ||
+                 s->inflight_migration; // FIXME: runstate check?

Without setting force_stop in vhost_user_blk_stop() I don't see how
the GET_VRING_BASE operation would be skipped.
Nothing skip GET_VRING_BASE) In the first versions of the patch, this was done, but then I realized that this was the wrong way, because the last_avail_idx and avail_idx counters may be incorrect after migration.
Therefore, it is now implemented that on GET_VRING_BASE the backend needs to either wait for all in-flight requests or not (depending on protocol_feature). 


On Thu, Jan 15, 2026 at 3:13 AM Alexandr Moshkov
<[email protected]> 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<[email protected]>
Reviewed-by: Stefan Hajnoczi<[email protected]>
---
  hw/block/vhost-user-blk.c          | 27 +++++++++++++++++++++++++++
  include/hw/virtio/vhost-user-blk.h |  1 +
  include/hw/virtio/vhost.h          |  6 ++++++
  3 files changed, 34 insertions(+)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index 4d81d2dc34..c151e83677 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -353,6 +353,7 @@ 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) {
@@ -568,6 +569,26 @@ 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 = virtio_has_feature(s->dev.protocol_features,
+                               VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
+
+    return inflight_migration;
+}
+
+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),
+        VMSTATE_END_OF_LIST()
+    },
+};
+
  static const VMStateDescription vmstate_vhost_user_blk = {
      .name = "vhost-user-blk",
      .minimum_version_id = 1,
@@ -576,6 +597,10 @@ 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 const Property vhost_user_blk_properties[] = {
@@ -591,6 +616,8 @@ 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 8158d4673d..1e41a2bcdf 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;
  };

  #endif
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 08bbb4dfe9..89817bd848 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -554,4 +554,10 @@ static inline int vhost_load_backend_state(struct 
vhost_dev *dev, QEMUFile *f,
  }
  #endif

+extern const VMStateDescription vmstate_vhost_inflight_region;
+#define VMSTATE_VHOST_INFLIGHT_REGION(_field, _state) \
+    VMSTATE_STRUCT_POINTER(_field, _state, \
+                           vmstate_vhost_inflight_region, \
+                           struct vhost_inflight)
+
  #endif
--
2.34.1

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