Re: [Qemu-devel] [PATCH v3 3/4] xen: create qdev for each backend device

[Eduardo Habkost <ehabkost@redhat.com>]

Hi,

I am reviewing the existing code that sets has_dynamic_sysbus,
and this patch confused me. I have a question below:

On Tue, Nov 22, 2016 at 07:10:58AM +0100, Juergen Gross wrote:
> Create a qdev plugged to the xen-sysbus for each new backend device.
> This device can be used as a parent for all needed devices of that
> backend. The id of the new device will be "xen-<type>-<dev>" with
> <type> being the xen backend type (e.g. "qdisk") and <dev> the xen
> backend number of the type under which it is to be found in xenstore.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
> ---
>  hw/xen/xen_backend.c         | 47 ++++++++++++++++++++++++++++++++++++++++++++
>  hw/xen/xen_pvdev.c           |  4 +++-
>  include/hw/xen/xen_backend.h |  4 ++++
>  include/hw/xen/xen_pvdev.h   |  1 +
>  4 files changed, 55 insertions(+), 1 deletion(-)
> 
[...]
> +static void xen_set_dynamic_sysbus(void)
> +{
> +    Object *machine = qdev_get_machine();
> +    ObjectClass *oc = object_get_class(machine);
> +    MachineClass *mc = MACHINE_CLASS(oc);
> +
> +    mc->has_dynamic_sysbus = true;
> +}
> +

Why exactly is this code necessary? I assume there's no current
use case for "-device xen-sysdev" and "-device xen-backend",
correct? The devices created by C code like xen_be_get_xendev()
aren't supposed to be returned by
foreach_dynamic_sysbus_device().


>  int xen_be_register(const char *type, struct XenDevOps *ops)
>  {
>      char path[50];
> @@ -562,6 +579,8 @@ int xen_be_register(const char *type, struct XenDevOps *ops)
>  
>  void xen_be_register_common(void)
>  {
> +    xen_set_dynamic_sysbus();
> +
>      xen_be_register("console", &xen_console_ops);
>      xen_be_register("vkbd", &xen_kbdmouse_ops);
>      xen_be_register("qdisk", &xen_blkdev_ops);
[...]

-- 
Eduardo