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

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On 01/19/2012 09:56 AM, Luiz Capitulino wrote:
> 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...

A nice plus with this is we retroactively get support for a wide range 
of existing guest types that we wouldn't realistically expect to have a 
qemu-ga package, and I think there's even balloon stats for Windows 
guests as well. Event-based updates also lend themselves nicely to 
things like qemu-centric UI integration.

Memory stats is one of the proposed use-cases of qemu-ga, but I don't 
see these as necessarily being in conflict. We can extend the statistics 
collection quite a bit more with an agent approach, but it's nice to 
have some assurance that if you can balloon a guest, you can at the very 
least get at the bare minimum information you need to make reasonable 
decisions about ballooning.

>
>   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(-)
>