vhost-user-blk: support inter-host inflight migration

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>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.s.norwitz@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260115081103.655749-6-dtalexundeer@yandex-team.ru>

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)

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

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