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Re: [Qemu-devel] [PATCH v3] qdev/core: fix qbus_is_full()

From: Tony Krowiak
Subject: Re: [Qemu-devel] [PATCH v3] qdev/core: fix qbus_is_full()
Date: Mon, 28 Jan 2019 15:35:09 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 12/17/18 10:57 AM, Tony Krowiak wrote:
The qbus_is_full(BusState *bus) function (qdev_monitor.c) compares the max_index
value of the BusState structure with the max_dev value of the BusClass structure
to determine whether the maximum number of children has been reached for the
bus. The problem is, the max_index field of the BusState structure does not
necessarily reflect the number of devices that have been plugged into
the bus.

Whenever a child device is plugged into the bus, the bus's max_index value is
assigned to the child device and then incremented. If the child is subsequently
unplugged, the value of the max_index does not change and no longer reflects the
number of children.

When the bus's max_index value reaches the maximum number of devices
allowed for the bus (i.e., the max_dev field in the BusClass structure),
attempts to plug another device will be rejected claiming that the bus is
full -- even if the bus is actually empty.

To resolve the problem, a new 'num_children' field is being added to the
BusState structure to keep track of the number of children plugged into the
bus. It will be incremented when a child is plugged, and decremented when a
child is unplugged.

Signed-off-by: Tony Krowiak <address@hidden>
Reviewed-by: Pierre Morel<address@hidden>
Reviewed-by: Halil Pasic <address@hidden>
  hw/core/qdev.c         | 3 +++
  include/hw/qdev-core.h | 1 +
  qdev-monitor.c         | 2 +-
  3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index 6b3cc55b27c2..956923f33520 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -59,6 +59,8 @@ static void bus_remove_child(BusState *bus, DeviceState 
              snprintf(name, sizeof(name), "child[%d]", kid->index);
              QTAILQ_REMOVE(&bus->children, kid, sibling);
+ bus->num_children--;
              /* This gives back ownership of kid->child back to us.  */
              object_property_del(OBJECT(bus), name, NULL);
@@ -73,6 +75,7 @@ static void bus_add_child(BusState *bus, DeviceState *child)
      char name[32];
      BusChild *kid = g_malloc0(sizeof(*kid));
+ bus->num_children++;
      kid->index = bus->max_index++;
      kid->child = child;
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index a24d0dd566e3..521f0a947ead 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -206,6 +206,7 @@ struct BusState {
      HotplugHandler *hotplug_handler;
      int max_index;
      bool realized;
+    int num_children;
      QTAILQ_HEAD(ChildrenHead, BusChild) children;
      QLIST_ENTRY(BusState) sibling;
diff --git a/qdev-monitor.c b/qdev-monitor.c
index 07147c63bf8b..45a8ba49644c 100644
--- a/qdev-monitor.c
+++ b/qdev-monitor.c
@@ -414,7 +414,7 @@ static DeviceState *qbus_find_dev(BusState *bus, char *elem)
  static inline bool qbus_is_full(BusState *bus)
      BusClass *bus_class = BUS_GET_CLASS(bus);
-    return bus_class->max_dev && bus->max_index >= bus_class->max_dev;
+    return bus_class->max_dev && bus->num_children >= bus_class->max_dev;

Just checking back on this one. Do we want to merge this patch and deal
with the max_index issue in another patch, in this patch, or not at all?

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