[v2] vhost-user-blk: support inflight migration

[PATCH v2 3/3] vhost-user-blk: support inter-host inflight migration

Posted by Alexandr Moshkov 2 weeks, 2 days ago

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 force_stop = true,
so GET_VRING_BASE will not be executed.

Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
---
 hw/block/vhost-user-blk.c          | 30 ++++++++++++++++++++++++++++++
 include/hw/virtio/vhost-user-blk.h |  1 +
 2 files changed, 31 insertions(+)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index a8fd90480a..29d4505d24 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -242,6 +242,12 @@ static int vhost_user_blk_stop(VirtIODevice *vdev)
     force_stop = s->skip_get_vring_base_on_force_shutdown &&
                  qemu_force_shutdown_requested();
 
+    if (s->enable_inflight_region_migration &&
+        !migrate_local_vhost_user_blk() &&
+        runstate_check(RUN_STATE_FINISH_MIGRATE)) {
+        force_stop = true;
+    }
+
     s->dev.backend_transfer = s->dev.backend_transfer ||
         (runstate_check(RUN_STATE_FINISH_MIGRATE) &&
          migrate_local_vhost_user_blk());
@@ -656,6 +662,24 @@ 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->enable_inflight_region_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);
@@ -678,6 +702,10 @@ 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 +779,8 @@ 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("enable-inflight-migration", VHostUserBlk,
+                     enable_inflight_region_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..1556714296 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 enable_inflight_region_migration;
 
     bool incoming_backend;
 };
-- 
2.34.1

Re: [PATCH v2 3/3] vhost-user-blk: support inter-host inflight migration

Posted by Raphael Norwitz 1 week, 4 days ago

A couple of comments but overall looks good to me.

On Wed, Oct 29, 2025 at 5:43 AM Alexandr Moshkov
<dtalexundeer@yandex-team.ru> 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, 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 force_stop = true,
> so GET_VRING_BASE will not be executed.
>
> Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
> ---
>  hw/block/vhost-user-blk.c          | 30 ++++++++++++++++++++++++++++++
>  include/hw/virtio/vhost-user-blk.h |  1 +
>  2 files changed, 31 insertions(+)
>
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index a8fd90480a..29d4505d24 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -242,6 +242,12 @@ static int vhost_user_blk_stop(VirtIODevice *vdev)
>      force_stop = s->skip_get_vring_base_on_force_shutdown &&
>                   qemu_force_shutdown_requested();
>

Why not move the vhost_user_blk_inflight_needed() helper up here
instead and rather do:

if (vhost_user_blk_inflight_needed(s) &&
    runstate_check(RUN_STATE_FINISH_MIGRATE))

As is it looks like vhost_user_blk_inflight_needed() is unused.

> +    if (s->enable_inflight_region_migration &&
> +        !migrate_local_vhost_user_blk() &&
> +        runstate_check(RUN_STATE_FINISH_MIGRATE)) {
> +        force_stop = true;
> +    }
> +
>      s->dev.backend_transfer = s->dev.backend_transfer ||
>          (runstate_check(RUN_STATE_FINISH_MIGRATE) &&
>           migrate_local_vhost_user_blk());
> @@ -656,6 +662,24 @@ 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->enable_inflight_region_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);
> @@ -678,6 +702,10 @@ 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 +779,8 @@ 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),

I would prefer the name indicating that the goal of the parameter was
to skip the GET_VRING_BASE like
"skip-get-vring-base-on-force-shutdown".

Maybe rename it something like
"skip-get-vring-base-inflight-migration"/skip_get_viring_base_migrate_inflight?

> +    DEFINE_PROP_BOOL("enable-inflight-migration", VHostUserBlk,
> +                     enable_inflight_region_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..1556714296 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 enable_inflight_region_migration;
>
>      bool incoming_backend;
>  };
> --
> 2.34.1
>
>

[PATCH v2 1/3] vmstate: introduce VMSTATE_VBUFFER_UINT64
[PATCH v2 2/3] vhost: add vmstate for inflight region with inner buffer
[PATCH v2 3/3] vhost-user-blk: support inter-host inflight migration