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

[Juan Quintela <quintela@redhat.com>]

"Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Thu, Feb 16, 2023 at 11:11:22AM -0500, Michael S. Tsirkin wrote:
>> On Thu, Feb 16, 2023 at 03:14:05PM +0100, Juan Quintela wrote:
>> > 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.
>> 
>> meaning the cast? yes.
>> 
>> > 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()
>
> In fact why do we want to migrate this property?
> We generally don't, we only migrate state.

See previous discussion.
In a nutshell, we are going to have internal migration in the future
(not done yet).

Later, Juan.

>> > > +    },
>> > > +   .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.
>> 
>> Weird suggestion.  We generally don't do this kind of check - that
>> would be open-coding each property. It's management's job to make
>> sure things are consistent.
>> 
>> -- 
>> MST