[Qemu-devel] [RFC 0/5]: QMP: add balloon-get-memory-stats command

[Luiz Capitulino]

Long ago, commit 625a5be added the guest provided memory statistics to
the query-balloon command. Unfortunately, it also introduced a severe
bug: query-balloon would hang if the guest didn't respond. This, in turn,
would also cause a hang in libvirt.

Because of that, we decided to disable the guest memory stats feature
(commit 11724ff).

As we decided to let commands implement ad-hoc async mechanisms until we
get a proper way to do it, I decided to try to re-enable that feature.

My idea is to have a command and an event. The command gets the process
started by sending a request to guest and returns. Later, when the guest
makes the memory stats info available, it's sent to the client by means
of an QMP event (please, take a look at patch 05/05 for full details).

I'm not sure if that approach is good for libvirt though, so it would be
very helpful to get their input (Eric, I'm CC'ing you here, but feel free
to route this to someone else).

Another interesting point is that, there's another way of doing this and
it's using qemu-ga instead. That's, qemu-ga could read that information
from proc and return it. This is easier & simpler, as it doesn't involve
guest communication. We also could return a lot more information if needed.
The only disadvantage I can see is the dependency on qemu-ga...

 QMP/qmp-events.txt  |   28 ++++++++++++++++++++++++++++
 balloon.c           |   47 +++++++++++++++++++++++++++++++++++++----------
 balloon.h           |    7 ++++---
 hmp.c               |   25 +------------------------
 hw/virtio-balloon.c |   39 +++++++++++++++++++++++++++------------
 monitor.c           |    3 +++
 monitor.h           |    1 +
 qapi-schema.json    |   42 ++++++++++++++++++++++--------------------
 qmp-commands.hx     |    6 ++++++
 9 files changed, 129 insertions(+), 69 deletions(-)