Re: [RFC PATCH v2 2/3] vfio: Set the priority of the VFIO VM state change handler explicitly

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 9 Dec 2020 16:09:18 +0800
Shenming Lu <lushenming@huawei.com> wrote:

> In the VFIO VM state change handler, VFIO devices are transitioned
> in the _SAVING state, which should keep them from sending interrupts.
> Then we can save the pending states of all interrupts in the GIC VM
> state change handler (on ARM).
> 
> So we have to set the priority of the VFIO VM state change handler
> explicitly (like virtio devices) to ensure it is called before the
> GIC's in saving.

What this patch does is to make the priority of the vfio migration
state change handler depending on the position in the qdev tree. As all
state change handlers with no explicit priority are added at priority
0, this will make sure that this handler runs before (save) resp. after
(restore) nearly all other handlers, which will address your issue here
(and possibly similar ones).

So, this patch seems fine for now, but I'm wondering whether we need to
think more about priorities for handlers in general, and if there are
more hidden dependencies lurking in there.

> 
> Signed-off-by: Shenming Lu <lushenming@huawei.com>
> Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
> ---
>  hw/vfio/migration.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> index 3b9de1353a..97ea82b100 100644
> --- a/hw/vfio/migration.c
> +++ b/hw/vfio/migration.c
> @@ -862,7 +862,8 @@ static int vfio_migration_init(VFIODevice *vbasedev,
>      register_savevm_live(id, VMSTATE_INSTANCE_ID_ANY, 1, &savevm_vfio_handlers,
>                           vbasedev);
>  
> -    migration->vm_state = qemu_add_vm_change_state_handler(vfio_vmstate_change,
> +    migration->vm_state = qdev_add_vm_change_state_handler(vbasedev->dev,
> +                                                           vfio_vmstate_change,
>                                                             vbasedev);
>      migration->migration_state.notify = vfio_migration_state_notifier;
>      add_migration_state_change_notifier(&migration->migration_state);