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

[Michael S. Tsirkin]

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()
> > +    },
> > +   .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.

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MST