The main reason of adding a new type is to make cross-device live migration
between "virtio-blk" and "vhost-user-blk" devices possible in both directions.

It might be useful for the cases when a slow block layer should be replaced
with a more performant one on running VM without stopping, i.e. with very low
downtime comparable with the one on migration.

It's possible to achive that for two reasons:

1.The VMStates of "virtio-blk" and "vhost-user-blk" are almost the same.
  They consist of the identical VMSTATE_VIRTIO_DEVICE and differs from
  each other in the values of migration service fields only.
2.The device driver used in the guest is the same: virtio-blk

The new type uses virtio-blk VMState instead of vhost-user-blk specific
VMstate, also it implements migration save/load callbacks to be compatible
with migration stream produced by "virtio-blk" device.

Adding the new vhost-user-blk type instead of modifying the existing one
is convenent. It ease to differ the new virtio-blk-compatible vhost-user-blk
device from the existing non-compatible one using qemu machinery without any
other modifiactions. That gives all the variety of qemu device related
constraints out of box.

Signed-off-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>
---
 hw/block/vhost-user-blk.c          | 63 ++++++++++++++++++++++++++++++
 include/hw/virtio/vhost-user-blk.h |  2 +
 2 files changed, 65 insertions(+)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index ba13cb87e520..877fe54e891f 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -30,6 +30,7 @@
 #include "hw/virtio/virtio-access.h"
 #include "sysemu/sysemu.h"
 #include "sysemu/runstate.h"
+#include "migration/qemu-file-types.h"
 
 #define REALIZE_CONNECTION_RETRIES 3
 
@@ -612,9 +613,71 @@ static const TypeInfo vhost_user_blk_info = {
     .class_init = vhost_user_blk_class_init,
 };
 
+/*
+ * this is the same as vmstate_virtio_blk
+ * we use it to allow virtio-blk <-> vhost-user-virtio-blk migration
+ */
+static const VMStateDescription vmstate_vhost_user_virtio_blk = {
+    .name = "virtio-blk",
+    .minimum_version_id = 2,
+    .version_id = 2,
+    .fields = (VMStateField[]) {
+        VMSTATE_VIRTIO_DEVICE,
+        VMSTATE_END_OF_LIST()
+    },
+};
+
+static void vhost_user_virtio_blk_save(VirtIODevice *vdev, QEMUFile *f)
+{
+    /*
+     * put a zero byte in the stream to be compatible with virtio-blk
+     */
+    qemu_put_sbyte(f, 0);
+}
+
+static int vhost_user_virtio_blk_load(VirtIODevice *vdev, QEMUFile *f,
+                                      int version_id)
+{
+    if (qemu_get_sbyte(f)) {
+        /*
+         * on virtio-blk -> vhost-user-virtio-blk migration we don't expect
+         * to get any infilght requests in the migration stream because
+         * we can't load them yet.
+         * TODO: consider putting those inflight requests to inflight region
+         */
+        error_report("%s: can't load in-flight requests",
+                     TYPE_VHOST_USER_VIRTIO_BLK);
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
+static void vhost_user_virtio_blk_class_init(ObjectClass *klass, void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+    VirtioDeviceClass *vdc = VIRTIO_DEVICE_CLASS(klass);
+
+    /* override vmstate of vhost_user_blk */
+    dc->vmsd = &vmstate_vhost_user_virtio_blk;
+
+    /* adding callbacks to be compatible with virtio-blk migration stream */
+    vdc->save = vhost_user_virtio_blk_save;
+    vdc->load = vhost_user_virtio_blk_load;
+}
+
+static const TypeInfo vhost_user_virtio_blk_info = {
+    .name = TYPE_VHOST_USER_VIRTIO_BLK,
+    .parent = TYPE_VHOST_USER_BLK,
+    .instance_size = sizeof(VHostUserBlk),
+    /* instance_init is the same as in parent type */
+    .class_init = vhost_user_virtio_blk_class_init,
+};
+
 static void virtio_register_types(void)
 {
     type_register_static(&vhost_user_blk_info);
+    type_register_static(&vhost_user_virtio_blk_info);
 }
 
 type_init(virtio_register_types)
diff --git a/include/hw/virtio/vhost-user-blk.h b/include/hw/virtio/vhost-user-blk.h
index 7c91f15040eb..d81f18d22596 100644
--- a/include/hw/virtio/vhost-user-blk.h
+++ b/include/hw/virtio/vhost-user-blk.h
@@ -23,6 +23,8 @@
 #include "qom/object.h"
 
 #define TYPE_VHOST_USER_BLK "vhost-user-blk"
+#define TYPE_VHOST_USER_VIRTIO_BLK "vhost-user-virtio-blk"
+
 OBJECT_DECLARE_SIMPLE_TYPE(VHostUserBlk, VHOST_USER_BLK)
 
 #define VHOST_USER_BLK_AUTO_NUM_QUEUES UINT16_MAX
-- 
2.25.1

On Mon, Oct 04, 2021 at 06:07:30PM +0300, Denis Plotnikov wrote:
> Adding the new vhost-user-blk type instead of modifying the existing one
> is convenent. It ease to differ the new virtio-blk-compatible vhost-user-blk
> device from the existing non-compatible one using qemu machinery without any
> other modifiactions. That gives all the variety of qemu device related
> constraints out of box.

Convenient for the developer maybe, but isn't it confusing for the user?
I don't really understand this paragraph. E.g. what are the constraints
here?



> 
> Signed-off-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>
> ---
>  hw/block/vhost-user-blk.c          | 63 ++++++++++++++++++++++++++++++
>  include/hw/virtio/vhost-user-blk.h |  2 +
>  2 files changed, 65 insertions(+)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index ba13cb87e520..877fe54e891f 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -30,6 +30,7 @@
>  #include "hw/virtio/virtio-access.h"
>  #include "sysemu/sysemu.h"
>  #include "sysemu/runstate.h"
> +#include "migration/qemu-file-types.h"
>  
>  #define REALIZE_CONNECTION_RETRIES 3
>  
> @@ -612,9 +613,71 @@ static const TypeInfo vhost_user_blk_info = {
>      .class_init = vhost_user_blk_class_init,
>  };
>  
> +/*
> + * this is the same as vmstate_virtio_blk
> + * we use it to allow virtio-blk <-> vhost-user-virtio-blk migration
> + */
> +static const VMStateDescription vmstate_vhost_user_virtio_blk = {
> +    .name = "virtio-blk",
> +    .minimum_version_id = 2,
> +    .version_id = 2,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_VIRTIO_DEVICE,
> +        VMSTATE_END_OF_LIST()
> +    },
> +};
> +
> +static void vhost_user_virtio_blk_save(VirtIODevice *vdev, QEMUFile *f)
> +{
> +    /*
> +     * put a zero byte in the stream to be compatible with virtio-blk
> +     */
> +    qemu_put_sbyte(f, 0);
> +}
> +
> +static int vhost_user_virtio_blk_load(VirtIODevice *vdev, QEMUFile *f,
> +                                      int version_id)
> +{
> +    if (qemu_get_sbyte(f)) {
> +        /*
> +         * on virtio-blk -> vhost-user-virtio-blk migration we don't expect
> +         * to get any infilght requests in the migration stream because
> +         * we can't load them yet.
> +         * TODO: consider putting those inflight requests to inflight region
> +         */
> +        error_report("%s: can't load in-flight requests",
> +                     TYPE_VHOST_USER_VIRTIO_BLK);
> +        return -EINVAL;
> +    }
> +
> +    return 0;
> +}
> +
> +static void vhost_user_virtio_blk_class_init(ObjectClass *klass, void *data)
> +{
> +    DeviceClass *dc = DEVICE_CLASS(klass);
> +    VirtioDeviceClass *vdc = VIRTIO_DEVICE_CLASS(klass);
> +
> +    /* override vmstate of vhost_user_blk */
> +    dc->vmsd = &vmstate_vhost_user_virtio_blk;

So can't we do something like this in vhost_user_blk_class_init:

	if qemu >= 6.2
		dc->vmsd = &vmstate_vhost_user_virtio_blk;
	else
		dc->vmsd = &vmstate_vhost_user_blk

?

> +
> +    /* adding callbacks to be compatible with virtio-blk migration stream */
> +    vdc->save = vhost_user_virtio_blk_save;
> +    vdc->load = vhost_user_virtio_blk_load;
> +}
> +
> +static const TypeInfo vhost_user_virtio_blk_info = {
> +    .name = TYPE_VHOST_USER_VIRTIO_BLK,
> +    .parent = TYPE_VHOST_USER_BLK,
> +    .instance_size = sizeof(VHostUserBlk),
> +    /* instance_init is the same as in parent type */
> +    .class_init = vhost_user_virtio_blk_class_init,
> +};
> +
>  static void virtio_register_types(void)
>  {
>      type_register_static(&vhost_user_blk_info);
> +    type_register_static(&vhost_user_virtio_blk_info);
>  }
>  
>  type_init(virtio_register_types)
> diff --git a/include/hw/virtio/vhost-user-blk.h b/include/hw/virtio/vhost-user-blk.h
> index 7c91f15040eb..d81f18d22596 100644
> --- a/include/hw/virtio/vhost-user-blk.h
> +++ b/include/hw/virtio/vhost-user-blk.h
> @@ -23,6 +23,8 @@
>  #include "qom/object.h"
>  
>  #define TYPE_VHOST_USER_BLK "vhost-user-blk"
> +#define TYPE_VHOST_USER_VIRTIO_BLK "vhost-user-virtio-blk"
> +
>  OBJECT_DECLARE_SIMPLE_TYPE(VHostUserBlk, VHOST_USER_BLK)
>  
>  #define VHOST_USER_BLK_AUTO_NUM_QUEUES UINT16_MAX
> -- 
> 2.25.1