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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: Thu, 10 Jan 2019 10:50:30 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 1/9/19 12:35 PM, Halil Pasic wrote:
On Wed, 9 Jan 2019 10:36:11 -0500
Tony Krowiak <address@hidden> wrote:

On 1/9/19 5:14 AM, Cornelia Huck wrote:
On Tue, 8 Jan 2019 15:34:37 -0500
Tony Krowiak <address@hidden> wrote:

On 1/8/19 12:06 PM, Cornelia Huck wrote:
On Tue, 8 Jan 2019 17:50:21 +0100
Halil Pasic <address@hidden> wrote:
On Tue, 8 Jan 2019 17:31:13 +0100
Cornelia Huck <address@hidden> wrote:
On Tue, 8 Jan 2019 11:08:56 -0500
Tony Krowiak <address@hidden> wrote:
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(-)

Ping ...
I could not determine who the maintainer is for the three files
listed above. I checked the MAINTAINERS file and the prologue of each
individual file. Can someone please tell me who is responsible
for merging these changes? Any additional review comments?

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++;

Hm... I'm wondering what happens for insane numbers of hotplugging
operations here?

(Preexisting problem for busses without limit, but busses with a limit
could now run into that as well.)

How does this patch change things? I mean bus->num_children gets
decremented on unplug.

We don't stop anymore if max_index >= max_dev, which means that we can
now trigger that even if max_dev != 0.

I guess I am missing your point. If max_dev == 0, then there is nothing
stopping an insane number of hot plug operations; either before this
patch, or with this patch. With the patch, once the number of children
hot plugged reaches max_dev, the qbus_is_full function will return false
and no more children can be plugged. If a child device is unplugged,
the num_children - which counts the number of children attached to the
bus - will be decremented, so it always reflects the number of children
added to the bus. Besides, checking max_index against max_dev
is erroneous, because max_index is incremented every time a child device
is plugged and is never decremented. It really operates as more of a
uinique identifier than a counter and is also used to create a unique
property name when the child device is linked to the bus as a property
(see bus_add_child function in hw/core/qdev.c).

Checking num_children against max_dev is the right thing to do, no
argument here.

However, max_index continues to be incremented even if devices have
been unplugged again. That means it can overflow, as it is never bound
by the max_dev constraint.

This has been a problem before for busses with an unrestricted number of
devices before, but with your patch, the problem is now triggerable for
all busses.

Not a problem with your patch, but we might want to look into making
max_index overflow/wraparound save.

I see your point. It does beg the question, what are the chances that
max_index reaches INT_MAX? In the interest of making this code more
bullet proof, I suppose it is something that should be dealt with.

A search reveals that max_index is used in only two places: It is used
to set the index for a child of the bus (i.e., bus_add_child function in
hw/core/qdev.c); and prior to this patch, to determine if max_dev has
been exceeded (i.e., qbus_is_full function in qdev_monitor.c). From
what I can see, the bus child index is used only to generate a property
name of the format "child[%d]" so the child can be linked as a property
of the bus (see bus_add_child and bus_remove_child functions in
hw/core/qdev.c). Wraparound of the max_index would most likely result in
generating a duplicate property name for the child.

I propose two possible solutions:

1. When max_index reaches INT_MAX, do not allow any additional children
     to be added to the bus.

2. Set a max_dev value of INT_MAX for the BusClass instance if the value
     is not set (in the bus_class_init function in hw/core/bus.c).

3. Instead of generating the property name from the BusChild index
     value, generate a UUID string. Since the index field of the BusChild
     appears to be used only to generate the child's name, maybe change
     the index field to a UUID field or a name field.

Separate problem, separate patch, or?

Good question.

UUID instead of index solves the problem of unique names I guess, but I
can't tell if that would be acceptable (interface stability, etc.).

I may have missed something, but as currently used, the BusChild index
is accessed in only two places; when the child is added to the bus and
when it is removed from the bus. In both cases, it is used to derive
a unique property name for the child device.

Speaking of index, it implies ordering of the bus's children. IMHO, it
only makes semantical sense if the index changes when a child device
with a lower index value is removed from the bus. If a child device
has an index of 5 - i.e., the fifth child on the bus - and the child
device with index 4 is removed, then the child device with index 5 is
no longer the fifth child on the bus. Maybe its just the fact that
these fields are named 'index'. The fact that they are not really used
as indexes caused a bit of confusion for me when analyzing this code and
seems like a misnomer to me.

The max_dev won't help because we can get a collision while maintaining
the invariant 'not more than 2 devices on the bus'.

I don't understand, can you better explain what you mean? When you
say "we can get a collision", are you talking about the property
name? What does max_dev have to do with that? Please explain. What do
you mean by "maintaining the invariant 'not more than 2 devices on the

So if we don't want to limit the number of hotplug operations, and we do
want to keep the allocation scheme before wrapping, the only solution I
see is looking for the first free identifier after we wrap.

Are you are saying to look for the first index value that is not
assigned to a BusChild object?

BTW I wonder if making max_index and index unsigned make things bit less

I thought the same. They could also be made unsigned long or
unsigned long long to increase the number of child devices that can be
plugged in before having to deal with exceeding the index value.


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