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, the vhost-user backend can cancel all
inflight requests and
then, after migration, they will be executed on another host.

In vhost_user_blk_stop() on incoming inter-host migration make
should_drain = true, so after GET_VRING_BASE message all in-flight
requests will be migrated to other vm

Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
---
 hw/block/vhost-user-blk.c          | 25 +++++++++++++++++++++++++
 include/hw/virtio/vhost-user-blk.h |  1 +
 2 files changed, 26 insertions(+)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index 56d55c18c8..c537e7ef9b 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -242,6 +242,7 @@ static int vhost_user_blk_stop(VirtIODevice *vdev)
 
     force_stop = s->skip_get_vring_base_on_force_shutdown &&
                  qemu_force_shutdown_requested();
+    should_drain = !s->inflight_migration;
 
     s->dev.backend_transfer = s->dev.backend_transfer ||
         (runstate_check(RUN_STATE_FINISH_MIGRATE) &&
@@ -657,6 +658,24 @@ 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;
+
+    return s->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),
+        VMSTATE_END_OF_LIST()
+    },
+};
+
 static bool vhost_user_blk_pre_incoming(void *opaque, Error **errp)
 {
     VHostUserBlk *s = VHOST_USER_BLK(opaque);
@@ -679,6 +698,10 @@ 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)
@@ -752,6 +775,8 @@ 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