From: Paul Durrant <pdurrant@amazon.com>

'xen-sysdev' plugs into the 'System' bus type, not 'xen-sysbus. That bus type
is what 'xen-backend' plugs into.
'xen-sysdev' is drived form 'sys-bus-device' so the bus type need not be
overridden. 'xen-backend' is derived from 'device', which plugs into the
generic 'bus' type, so its bus type should be overridden to 'xen-sysbus'.

Without this patch, the following assertion will fail:

qemu-system-i386: hw/core/qdev.c:102: qdev_set_parent_bus: Assertion
`dc->bus_type && object_dynamic_cast(OBJECT(bus), dc->bus_type)'
failed.

Reported-by: Jason Andryuk <jandryuk@gmail.com>
Fixes: 81cb05732efb ("qdev: Assert devices are plugged into a bus that can take them")
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
---
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
---
 hw/xen/xen-legacy-backend.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/xen/xen-legacy-backend.c b/hw/xen/xen-legacy-backend.c
index 2335ee2e65..c5c75c0064 100644
--- a/hw/xen/xen-legacy-backend.c
+++ b/hw/xen/xen-legacy-backend.c
@@ -789,11 +789,12 @@ static void xendev_class_init(ObjectClass *klass, void *data)
     set_bit(DEVICE_CATEGORY_MISC, dc->categories);
     /* xen-backend devices can be plugged/unplugged dynamically */
     dc->user_creatable = true;
+    dc->bus_type = TYPE_XENSYSBUS;
 }
 
 static const TypeInfo xendev_type_info = {
     .name          = TYPE_XENBACKEND,
-    .parent        = TYPE_XENSYSDEV,
+    .parent        = TYPE_DEVICE,
     .class_init    = xendev_class_init,
     .instance_size = sizeof(struct XenLegacyDevice),
 };
@@ -824,7 +825,6 @@ static void xen_sysdev_class_init(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     device_class_set_props(dc, xen_sysdev_properties);
-    dc->bus_type = TYPE_XENSYSBUS;
 }
 
 static const TypeInfo xensysdev_info = {
-- 
2.20.1

On 6/24/20 2:18 PM, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> 'xen-sysdev' plugs into the 'System' bus type, not 'xen-sysbus. That bus type
> is what 'xen-backend' plugs into.
> 'xen-sysdev' is drived form 'sys-bus-device' so the bus type need not be
> overridden. 'xen-backend' is derived from 'device', which plugs into the
> generic 'bus' type, so its bus type should be overridden to 'xen-sysbus'.
> 
> Without this patch, the following assertion will fail:
> 
> qemu-system-i386: hw/core/qdev.c:102: qdev_set_parent_bus: Assertion
> `dc->bus_type && object_dynamic_cast(OBJECT(bus), dc->bus_type)'
> failed.
> 
> Reported-by: Jason Andryuk <jandryuk@gmail.com>
> Fixes: 81cb05732efb ("qdev: Assert devices are plugged into a bus that can take them")
> Signed-off-by: Paul Durrant <pdurrant@amazon.com>
> ---
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> ---
>  hw/xen/xen-legacy-backend.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/xen/xen-legacy-backend.c b/hw/xen/xen-legacy-backend.c
> index 2335ee2e65..c5c75c0064 100644
> --- a/hw/xen/xen-legacy-backend.c
> +++ b/hw/xen/xen-legacy-backend.c
> @@ -789,11 +789,12 @@ static void xendev_class_init(ObjectClass *klass, void *data)
>      set_bit(DEVICE_CATEGORY_MISC, dc->categories);
>      /* xen-backend devices can be plugged/unplugged dynamically */
>      dc->user_creatable = true;
> +    dc->bus_type = TYPE_XENSYSBUS;
>  }
>  
>  static const TypeInfo xendev_type_info = {
>      .name          = TYPE_XENBACKEND,
> -    .parent        = TYPE_XENSYSDEV,
> +    .parent        = TYPE_DEVICE,
>      .class_init    = xendev_class_init,
>      .instance_size = sizeof(struct XenLegacyDevice),
>  };
> @@ -824,7 +825,6 @@ static void xen_sysdev_class_init(ObjectClass *klass, void *data)
>      DeviceClass *dc = DEVICE_CLASS(klass);
>  
>      device_class_set_props(dc, xen_sysdev_properties);
> -    dc->bus_type = TYPE_XENSYSBUS;
>  }
>  
>  static const TypeInfo xensysdev_info = {
> 

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>