libvirt users - Dec 2021 - Qemu monitor info tlb gives unable to encode message payload

Hi,

the command
    virsh qemu-monitor-command ubuntu_vm --hmp --cmd "info tlb"
fails with "error: Unable to encode message payload".

I found a bugtracker entry for a similiar error [1], but I don't if this is
the same error (message too large). I also don't know how large an info tlb
message is.
Preferably I would not have to recompile libvirt just to issue this monitor
command..

Then I thought about circumventing the error by connecting directly to the qemu
monitor via netcat, but I found a thread [2] that says I cannot add my own
"-monitor tcp:..." to the Qemu commandline arguments.

Could someone help me with this?

Best,
Philipp

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1440683
[2]
https://libvirt-users.redhat.narkive.com/E0UTmdhU/is-there-a-way-to-disable-libvirt-qemu-monitor-while-creating-kvm-vm-using-virsh

On Wed, Dec 08, 2021 at 10:30:27 +0000, Philipp Klocke
wrote:
> Hi,
> 
> the command
>     virsh qemu-monitor-command ubuntu_vm --hmp --cmd "info tlb"
> fails with "error: Unable to encode message payload".
> 
> I found a bugtracker entry for a similiar error [1], but I don't if
this is the same error (message too large). I also don't know how large an
info tlb message is.
> Preferably I would not have to recompile libvirt just to issue this monitor
command..
Libvirt unfortunately limits strings to 4MiB:

const REMOTE_STRING_MAX = 4194304;

And the reply from qemu-monitor-command is a single string. Now
internally we process JSON messages up to 10 MiB so one could argue that
we should increase the size for the 'qemu-monitor-command' reply up to
10MiB. This could be straightforward but it's questionable whether it's
worth it.
> Then I thought about circumventing the error by connecting directly to the
qemu monitor via netcat, but I found a thread [2] that says I cannot add my own
"-monitor tcp:..." to the Qemu commandline arguments.
IIRC at that point qemu wasn't able to handle two monitor connections.
At this point it is possible to have two concurrent connections to the
montitor. Obviously things may break and you get to keep the pieces if
it breaks.

By adding:

  <qemu:commandline>
    <qemu:arg value='-qmp'/>
    <qemu:arg value='tcp:127.0.0.1:1235'/>
  </qemu:commandline>


I then get a connection from qemu on the socket when starting the VM:

  $ nc -l -p 1235
  {"QMP": {"version": {"qemu": {"micro":
93, "minor": 1, "major": 6}, "package":
"v6.2.0-rc3"}, "capabilities": ["oob"]}}

I can then start conversing with the monitor:

  {"execute":"qmp_capabilities"}
  {"return": {}}
 
{"execute":"human-monitor-command","arguments":{"command-line":"info"}}
  {"return": "info balloon  - ....


Out of curiousity, what do you specifically need 'info tlb' for?