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..5e44f6253c 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -656,6 +656,28 @@ 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_drain = vhost_dev_has_feature(&s->dev,
+                        VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
+
+    return s->inflight_migration &&
+           inflight_drain &&
+           !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);
@@ -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

On Mon, Jan 12, 2026 at 04:45:03PM +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..5e44f6253c 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -656,6 +656,28 @@ 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_drain = vhost_dev_has_feature(&s->dev,
> +                        VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);

VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT must only be negotiated
when inflight_migration is enabled. Otherwise the backend will use this
feature even though vhost_user_blk_inflight_needed() skips migrating the
in-flight region.

On 1/12/26 23:19, Stefan Hajnoczi wrote:
> On Mon, Jan 12, 2026 at 04:45:03PM +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..5e44f6253c 100644
>> --- a/hw/block/vhost-user-blk.c
>> +++ b/hw/block/vhost-user-blk.c
>> @@ -656,6 +656,28 @@ 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_drain = vhost_dev_has_feature(&s->dev,
>> +                        VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
> VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT must only be negotiated
> when inflight_migration is enabled. Otherwise the backend will use this
> feature even though vhost_user_blk_inflight_needed() skips migrating the
> in-flight region.
Oh, I understand now. I'll fix this too, thanks!