On 12/8/21 14:15, Daniel P. Berrangé wrote: > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe. > > > On Wed, Dec 08, 2021 at 01:32:48PM +0100, Philipp Klocke wrote: >> On 12/8/21 12:09, Peter Krempa wrote: >>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe. >>> >>> >>> 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. >> Thanks for the clarification! I will try to go for the 2nd monitor then. >>>> 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> >> >> When I add this to the config via virsh edit and then do a shutdown + >> reboot, I get a kernel panic. >> I put the corresponding dmesg log on gist: >> https://gist.github.com/klockeph/323f3e0d23a3254ef98a65fd7c8f5a1c > > I can't see how that is related to adding an extra -qmp parameter, > since it doesn't affect the guest ABI > Thanks, it seems that the panic was unrelated and something else that I need to fix. > FWIW, exposing -qmp over a tcp socket is totally insecure - any > local process can connect to that. > For some reason the tcp connection didn't work, but unix socket does the trick. I can now issue "info tlb" JSON commands through this socket and get a reply. <qemu:commandline> <qemu:arg value='-qmp'/> <qemu:arg value='unix:/path-to/ubuntu_vm.sock,server,nowait'/> </qemu:commandline> Thanks everybody for the help! Best, Philipp
