Re: [Qemu-devel] Missing guest clock-sync on Host clock change

[Laszlo Ersek <lersek@redhat.com>]

On 03/27/14 09:41, Erik Rull wrote:
> Hi all,
> 
> I would like to have the guest "drifting" to a new set clock on the host.
> 
> My problem is the following:
> 
> - Host System (Linux) starts up, hwclock and kernel time are synced,
> guest starts up with -rtc clock=host,driftfix=slew (which I assume
> should fix any drift issue on ACPI compatible guest OSes)
> - Host System kernel time drifts against the hwclock (jiffies timer due
> to no other available useful timer on SMP systems - core2duo has no hpet!)
> - calling "hwclock -s" on the host resyncs the kernel time with the
> hwclock, so "date" and "hwclock" show the same again
> - the guest stays at the "old" kernel time before the sync - also after
> 1 hour the delta is still the same, so no sync or slew is done :-(
> 
> My guest OS is Windows 8, which must have ACPI enabled, otherwise it
> will not work.
> 
> Any ideas how to proceed? Maybe some command line parameters are wrong?
> 
> I need this resync for the guest due to external synchronization - it
> must not be millisecond-precise, but a 9 seconds shift during a run
> overnight is too much!

My take: the hardware clock (the RTC) in the guest has correct value,
but the guest OS system time os not refreshed from it. Install the guest
agent in Windows, and call its "guest-set-time" command (with virsh
qemu-agent-command, or otherwise). Do not pass any argument for the
optional "time" parameter; this way the guest will sync its kernel time
from its RTC.

See:
- qga/qapi-schema.json, "guest-set-time",
- qga/commands-win32.c, qmp_guest_set_time()

In any case this is just a guess.

Laszlo