Here is an suitable impl

http://lists.nongnu.org/archive/html/qemu-devel/2012-05/msg02215.html

It would also need the 'query-events' command to let libvirt detect whether the balloon event is present

https://lists.gnu.org/archive/html/qemu-devel/2012-05/msg02255.html

Daniel, are you going to keep pursuing this upstream? If you will, then I think this bz has to be re-assigned to you, because a backport won't be needed (ie. this will be automatically in RHEL7.0 if merged before QEMU 1.2).

Yes, I aim to continue posting updates upstream.  Depending on the final result, I may ultimately request a backport for RHEL-6, but I'll create a separate BZ for that when required.

Ok, re-assigning to you then. If this misses the 1.2 deadline for whatever reason, then you can re-assign it back to me for the RHEL7.0 backport work.

V2 of the patches, with rate limiting added

http://lists.nongnu.org/archive/html/qemu-devel/2012-05/msg02833.html

All 3 patches now accepted upstream in Luiz' QMP tree

commit 4860853d60ecea44b65e9cdefce980de3a641dce
Author: Daniel P. Berrange <berrange>
Date:   Mon May 21 17:59:51 2012 +0100

    Add 'query-events' command to QMP to query async events


commit 1bcf107bbae883540f45e11e9fdc912a496c3094
Author: Daniel P. Berrange <berrange>
Date:   Wed May 16 11:01:58 2012 +0100

    Add event notification for guest balloon changes


commit d218a99b8d590224229969594e3411e940a77a7b
Author: Daniel P. Berrange <berrange>
Date:   Mon May 21 17:27:20 2012 +0100

    Add rate limiting of RTC_CHANGE, BALLOON_CHANGE & WATCHDOG events

Fixed in qemu-kvm-1.4.0-1.el7

Test this bug on latest version:
Host:
# uname -r
3.10.0-79.el7.x86_64
# rpm -q qemu-kvm
qemu-kvm-1.5.3-41.el7.x86_64

steps:
1.boot guest with 
 ...-device virtio-balloon-pci,bus=pci.0,id=balloon0

2.QMP
# telnet 10.66.83.192 4444
{"execute":"query-balloon"}
{"return": {"actual": 2147483648}}
{"execute":"balloon","arguments":{"value":900000}}
{"return": {}}
{"timestamp": {"seconds": 1390717029, "microseconds": 460666}, "event": "BALLOON_CHANGE", "data": {"actual": 2146435072}}
{"timestamp": {"seconds": 1390717030, "microseconds": 460216}, "event": "BALLOON_CHANGE", "data": {"actual": 455081984}}
{"timestamp": {"seconds": 1390717030, "microseconds": 752577}, "event": "BALLOON_CHANGE", "data": {"actual": 300941312}}

3.{"execute":"query-events"}

Results:
After step 2, when change balloon size ,added the balloon events
After step 3,will see the balloon event:
...{"name": "BALLOON_CHANGE"},...{"name": "RTC_CHANGE"}..{"name": "WATCHDOG"}...

Test on old version:
Host:
# uname -r
3.7.0-0.36.el7.x86_64
# rpm -q qemu-kvm
qemu-kvm-1.3.0-3.el7.x86_64

i can get the same results.

Addtional test:

Also test change balloon size too small and big on latest version, not hit any problem.

Hi, Daniel

   According to above test , could we can verify pass this bug ?

tanks

When the balloon target is set, the guest will slowly adjust to meat the target. This will result in a series of BALLOON_CHANGE events. You should wait for the last event to arrive, and then compare the value with the guest RAM shown in /proc/meminfo inside the guest. They should be pretty much the same, if not identical.

(In reply to Daniel Berrange from comment #11)
> When the balloon target is set, the guest will slowly adjust to meat the
> target. This will result in a series of BALLOON_CHANGE events. You should
> wait for the last event to arrive, and then compare the value with the guest
> RAM shown in /proc/meminfo inside the guest. They should be pretty much the
> same, if not identical.


yes ,test on latest version

steps:
1.Boot guest with 
/usr/libexec/qemu-kvm -m 4G ....-device virtio-balloon-pci,bus=pci.0,id=balloon0

2.Change balloon size 
# telnet 10.66.83.192 4445
Trying 10.66.83.192...
Connected to 10.66.83.192.
Escape character is '^]'.
{"QMP": {"version": {"qemu": {"micro": 3, "minor": 5, "major": 1}, "package": " (qemu-kvm-1.5.3-41.el7)"}, "capabilities": []}}
{"execute":"qmp_capabilities"}
{"return": {}}
{"execute":"query-balloon"}                      
{"return": {"actual": 4294967296}}
{"execute":"balloon","arguments":{"value":1073741824}}
{"return": {}}
{"timestamp": {"seconds": 1390995440, "microseconds": 402484}, "event": "BALLOON_CHANGE", "data": {"actual": 4293918720}}
{"timestamp": {"seconds": 1390995441, "microseconds": 402074}, "event": "BALLOON_CHANGE", "data": {"actual": 2600468480}}
{"timestamp": {"seconds": 1390995442, "microseconds": 279347}, "event": "BALLOON_CHANGE", "data": {"actual": 1073741824}}
{"execute":"query-balloon"}
{"return": {"actual": 1073741824}}

3.In guest
# cat /proc/meminfo
MemTotal:         770748 kB
MemFree:          503988 kB
Buffers:            1468 kB
...

4.extend the balloon size
{"execute":"balloon","arguments":{"value":  2147483648}}
{"timestamp": {"seconds": 1390995917, "microseconds": 311927}, "event": "BALLOON_CHANGE", "data": {"actual": 806354944}}
{"timestamp": {"seconds": 1390995917, "microseconds": 837996}, "event": "BALLOON_CHANGE", "data": {"actual": 2147483648}}

4.In guest
# cat /proc/meminfo
MemTotal:        1819324 kB
MemFree:         1552348 kB
Buffers:            1468 kB
Cached:            98656 kB
SwapCached:            0 kB
Active:            97880 kB
....


According to above test ,when balloon size change,will show the balloon events ,and the mem size in guest (cat /proc/meminfo) will change too.


Hi,Daniel
    Could we can verify this bug now?

Looks reasonable to me.

This request was resolved in Red Hat Enterprise Linux 7.0.

Contact your manager or support representative in case you have further questions about the request.