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.
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
>
>
