Re: [PATCH v2 1/1] vhost-user-fs: add property to allow migration

[Juan Quintela]

Anton Kuchin <antonkuchin@yandex-team.ru> wrote:
> Now any vhost-user-fs device makes VM unmigratable, that also prevents
> qemu update without stopping the VM. In most cases that makes sense
> because qemu has no way to transfer FUSE session state.
>
> But it is good to have an option for orchestrator to tune this according to
> backend capabilities and migration configuration.
>
> This patch adds device property 'migration' that is 'none' by default
> to keep old behaviour but can be set to 'external' to explicitly allow
> migration with minimal virtio device state in migration stream if daemon
> has some way to sync FUSE state on src and dst without help from qemu.
>
> Signed-off-by: Anton Kuchin <antonkuchin@yandex-team.ru>

Reviewed-by: Juan Quintela <quintela@redhat.com>

The migration bits are correct.

And I can think a better way to explain that one device is migrated
externally.

If you have to respin:

> +static int vhost_user_fs_pre_save(void *opaque)
> +{
> +    VHostUserFS *fs = (VHostUserFS *)opaque;

This hack is useless.
I know that there are still lots of code that still have it.


Now remember that I have no clue about vhost-user-fs.

But this looks fishy
>  static const VMStateDescription vuf_vmstate = {
>      .name = "vhost-user-fs",
> -    .unmigratable = 1,
> +    .minimum_version_id = 0,
> +    .version_id = 0,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_VIRTIO_DEVICE,
> +        VMSTATE_UINT8(migration_type, VHostUserFS),
> +        VMSTATE_END_OF_LIST()
> +    },
> +   .pre_save = vhost_user_fs_pre_save,
>  };
>  
>  static Property vuf_properties[] = {
> @@ -309,6 +337,10 @@ static Property vuf_properties[] = {
>      DEFINE_PROP_UINT16("num-request-queues", VHostUserFS,
>                         conf.num_request_queues, 1),
>      DEFINE_PROP_UINT16("queue-size", VHostUserFS, conf.queue_size, 128),
> +    DEFINE_PROP_UNSIGNED("migration", VHostUserFS, migration_type,
> +                         VHOST_USER_MIGRATION_TYPE_NONE,
> +                         qdev_prop_vhost_user_migration_type,
> +                         uint8_t),
>      DEFINE_PROP_END_OF_LIST(),

We have four properties here (5 with the new migration one), and you
only migrate one.

This looks fishy, but I don't know if it makes sense.
If they _have_ to be configured the same on source and destination, I
would transfer them and check in post_load that the values are correct.

Later, Juan.