Re: [Qemu-devel] [PATCH] qdev/core: Can not replug device on bus that allows one device

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 10 Dec 2018 14:14:14 -0500
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> If the maximum number of devices allowed on a bus is 1 and a device
> which is plugged into the bus is subsequently unplugged, attempting to replug
> the device fails with error "Bus 'xxx' does not support hotplugging".
> The "error" is detected in the qbus_is_full(BusState *bus) function
> (qdev_monitor.c) because bus->max_index >= bus_class->max_dev. The
> root of the problem is that the bus->max_index is not decremented when a device
> is unplugged from the bus. This patch fixes that problem.
> 
> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> ---
>  hw/core/qdev.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> index 6b3cc55b27c2..b35b0bf27925 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -59,6 +59,8 @@ static void bus_remove_child(BusState *bus, DeviceState *child)
>              snprintf(name, sizeof(name), "child[%d]", kid->index);
>              QTAILQ_REMOVE(&bus->children, kid, sibling);
>  
> +            bus->max_index--;
that might cause problem when bus is allowed to has more than 1 device
and unplugged device is not the last one.
In that case bus_add_child() might create a child with duplicate name.

> +
>              /* This gives back ownership of kid->child back to us.  */
>              object_property_del(OBJECT(bus), name, NULL);
>              object_unref(OBJECT(kid->child));