On Tue, Jan 13, 2026 at 02:58:19PM +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 <[email protected]>
> ---
> 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..9093e98841 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -377,6 +377,7 @@ static int vhost_user_blk_init(DeviceState *dev, bool
> connect, 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,
> false, errp);
> if (ret < 0) {
> @@ -656,6 +657,27 @@ 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 = vhost_dev_has_feature(&s->dev,
> + VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
> +
> + return inflight_migration &&
> + !migrate_local_vhost_user_blk();
Here's the spot that should depend on migrate_local_vhost_user_blk() from
Vladimilr's RFC patch (again, likely to be renamed..).
Btw, is this check correct against "!migrate_local_vhost_user_blk()"? I
was expecting the feature off only if local=on, so I expect it to be:
return 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),
One other trivial nitpick while glimpsing over the patch: should we move
the macro definition from previous patch to this one, where it is used?
> + 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
>
--
Peter Xu
