[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From: Tony Krowiak
Subject: Re: [Qemu-devel] [PATCH] qdev/core: Can not replug device on bus that allows one device
Date: Thu, 13 Dec 2018 11:10:15 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 12/13/18 8:03 AM, Halil Pasic wrote:
On Thu, 13 Dec 2018 13:09:59 +0100
Igor Mammedov <address@hidden> wrote:

On Tue, 11 Dec 2018 14:41:00 -0500
Tony Krowiak <address@hidden> wrote:

On 12/11/18 10:23 AM, Igor Mammedov wrote:
On Mon, 10 Dec 2018 14:14:14 -0500
Tony Krowiak <address@hidden> 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 <address@hidden>
   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 
               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.

I see what you are saying. The max_index is assigned to the child device
index in the bus_add_child() function. The child device index is used to
generate a unique name for the child device. The generated name is then
used to link the child as a property to the bus. When the child is
removed from the bus, the name is regenerated from the child device
index and the named property is . It would therefore appear that the
real purpose of the max_index is to generate indexes for children of
the bus thus ensuring each child has a unique index. In other words,
the max_index value is only tangentially connected to indexing the list
of children.

This results in a disconnect between the usage of the max_index value
when adding and removing a child from the bus, and the check in the
qbus_is_full() function which compares the max_index to the maximum
number of devices allowed on the bus. If a child has been removed from
the bus, the max_index value does not indicate whether the bus is
full, it only specifies the index to be assigned to the next child to be
added to the bus.

To resolve this problem, I propose the following:

Add the following field to struct BusState (include/hw/qdev_core.h):

struct BusState {
      Object obj;
      DeviceState *parent;
      char *name;
      HotplugHandler *hotplug_handler;
      int max_index;
      bool realized;
+    int num_children;
      QTAILQ_HEAD(ChildrenHead, BusChild) children;
      QLIST_ENTRY(BusState) sibling;

Add the following lines of code to the add/remove child functions in

static void bus_add_child(BusState *bus, DeviceState *child)
      char name[32];
      BusChild *kid = g_malloc0(sizeof(*kid));

      kid->index = bus->max_index++;
      kid->child = child;

      QTAILQ_INSERT_HEAD(&bus->children, kid, sibling);

      /* This transfers ownership of kid->child to the property.  */
      snprintf(name, sizeof(name), "child[%d]", kid->index);
      object_property_add_link(OBJECT(bus), name,
                               (Object **)&kid->child,
                               NULL, /* read-only property */
                               0, /* return ownership on prop deletion */

+    bus->num_children++;

static void bus_remove_child(BusState *bus, DeviceState *child)
      BusChild *kid;

      QTAILQ_FOREACH(kid, &bus->children, sibling) {
          if (kid->child == child) {
              char name[32];

              snprintf(name, sizeof(name), "child[%d]", kid->index);
              QTAILQ_REMOVE(&bus->children, kid, sibling);

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

+           bus->num_children--;


Change the line of code in the qbus_is_full() function in

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;

looks good to me

I agree, the second proposal looks reasonable. Can you send a proper
patch so I can r-b it?

will do


reply via email to

[Prev in Thread] Current Thread [Next in Thread]