When I set -rtc clock=vm the mig problem is solved, but what does this
flag exactly?

In the docu there is only

" If you want to isolate the guest time from the host, you can set clock to 
"rt" instead.
  To even prevent it from progressing during suspension, you can set it to 

what does this means?

Will the VM never be synced with the HW clock and so it will run slower
on cpu load on normal running?

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  windows hangs after live migration with virtio

Status in QEMU:

Bug description:
  Several of our users reported problems with windows machines hanging
  after live migrations. The common denominator _seems_ to be virtio
  I've managed to reproduce this reliably on a windows 10 (+
  virtio-win-0.1.118) guest, always within 1 to 5 migrations, with a
  virtio-scsi hard drive and a virtio-net network device. (When I
  replace the virtio-net device with an e1000 it takes 10 or more
  migrations, and without virtio devices I have not (yet) been able to
  reproduce this problem. I also could not reproduce this with a linux
  guest. Also spice seems to improve the situation, but doesn't solve
  it completely).

  I've tested quite a few tags from qemu-git (v2.2.0 through v2.6.1,
  and 2.6.1 with the patches mentioned on qemu-stable by Peter Lieven)
  and the behavior is the same everywhere.

  The reproducibility seems to be somewhat dependent on the host
  hardware, which makes investigating this issue that much harder.

  After the migration the windows graphics stack just hangs.
  Background processes are still running (eg. after installing an ssh
  server I could still login and get a command prompt after the hang was
  triggered... not that I'd know what to do with that on a windows
  machine...) - commands which need no GUI access work, the rest just
  hangs there on the command line, too.
  It's also capable of responding to an NMI sent via the qemu monitor:
  it then seems to "recover" and manages to show the blue sad-face
  screen that something happened, reboots successfully and is usable
  again without restarting the qemu process in between.
  From there whole the process can be repeated.

  Here's what our command line usually looks like:

  /usr/bin/qemu -daemonize \
        -enable-kvm \
        -chardev socket,id=qmp,path=/var/run/qemu-server/101.qmp,server,nowait 
-mon chardev=qmp,mode=control \
        -pidfile /var/run/qemu-server/101.pid \
        -smbios type=1,uuid=07fc916e-24c2-4eef-9827-4ab4026501d4 \
        -name win10 \
        -smp 6,sockets=1,cores=6,maxcpus=6 \
        -nodefaults \
        -boot menu=on,strict=on,reboot-timeout=1000 \
        -vga std \
        -vnc unix:/var/run/qemu-server/101.vnc \
        -no-hpet \
        -m 2048 \
        -device pci-bridge,id=pci.2,chassis_nr=2,bus=pci.0,addr=0x1f \
        -device pci-bridge,id=pci.1,chassis_nr=1,bus=pci.0,addr=0x1e \
        -device piix3-usb-uhci,id=uhci,bus=pci.0,addr=0x1.0x2 \
        -device usb-tablet,id=tablet,bus=uhci.0,port=1 \
        -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3 \
        -iscsi initiator-name=iqn.1993-08.org.debian:01:1ba48d46fb8 \
        -drive if=none,id=drive-ide0,media=cdrom,aio=threads \
        -device ide-cd,bus=ide.0,unit=0,drive=drive-ide0,id=ide0,bootindex=200 \
        -device virtio-scsi-pci,id=scsihw0,bus=pci.0,addr=0x5 \
        -rtc driftfix=slew,base=localtime \
        -global kvm-pit.lost_tick_policy=discard

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