[pve-devel] Windows live migration and time problems

[Wolfgang Bumiller]

Several users[1][2] have run into live migration issues with windows
machines which seems to have to do with windows' time handling[3][4].

According to a reply to [3] it seems to be possible to "fix" this by
enabling HPET (iow. removing the `-no-hpet` option) for windows
machines, which on the other hand might cause issues described in [4].

We've been able to reproduce this on both PVE and Ubuntu with libvirt.
In both cases we used a windows 10 machine with a virtio-scsi disk, 6
cores, 2G RAM and virtio-network. Changing any of these components
influences the probability of these hangs which makes it incredibly
hard to find a real solution.
The issue manifests in a hanging graphics stack. The machine still
responds to pings - if you have a command line open you might even be
able to type some commands there (ie. a `ping` to the outside works -
partially).

As for the time speed issue mentioned in [4], so far we've only
encountered this with windows XP. Curiously we start adding `-no-hpet`
with win vista/7 and up (same as libvirt does AFAIK, at least in my
tests I did see the -no-hpet option).
This makes me wonder if we should move this option to win-xp or
remove it altogether hoping that newer / more up to date windows
versions are less fragile in their time management? (Note that XP is
old, outdated and unsupported, so I don't care much about what happens
to it on a migration or how fast its clock is running... the setting
could be made optional in any case.)

It would be nice if some more people could test this and maybe give
some feedback (and possibly share their findings here on the qemu bug
tracker in [3] and/or [4]).

[1] https://forum.proxmox.com/threads/windows-vm-hang-after-live-migration.28216/#post-145128
[2] https://forum.proxmox.com/threads/live-migration-hungs-windows-guest.29101/
[3] https://bugs.launchpad.net/qemu/+bug/1618431
[4] https://bugs.launchpad.net/qemu/+bug/599958