[Qemu-devel] Re: [PATCH] re-set rtc date on reset handler

[Jan Kiszka <jan.kiszka@siemens.com>]

Jan Kiszka wrote:
> Blue Swirl wrote:
>> On Tue, Sep 1, 2009 at 7:22 PM, Glauber Costa<glommer@redhat.com> wrote:
>>> guests without a stable timesource such as kvm-clock will grab the wallclock
>>> from our rtc chip. However, we only sync the date when we first launch qemu.
>>> If a guest goes through a series of reboot cycles, it will slowly see time
>>> getting far behind the host.
>>>
>>> The proposal of this patch is to set the date to host clock again in the reset
>>> handler. With this patch, I see a Fedora guest keeping its clock in sync upon
>>> an ulimited number of reboots.
>> A different approach is used in m48t59.c: the guest clock is generated
>> directly from host clock without any timers and only a fixed offset
>> (to account for time when guest was stopped) is added, so the clock
>> will always in sync.
> 
> Ah, that looks like a useful approach! We currently face the issue of
> vRTC drifting away from the host time (as the latter is tuned by NTP).
> 
> Do you or anyone else know if switching the PC RTC over to the scheme
> used in the m48t59 may have some downsides? If not, I would happily hack
> up a patch ASAP and suggest it for mainline.

Clearly, one downside is that a jumping host time will cause the RTC to
jump as well. However, there might by setups where this does not happen,
so optionally switching the RTC over rt_clock seems like a reasonable
feature to me.

What about consolidating -localtime, -starttime and -rtc-td-hack at this
chance? Something like

 -rtc base=utc|localtime|date,clock=vm|rt

with 'date' specifying the base as in -starttime. 'clock' would then
allow to drive the RTC via the host's CLOCK_REALTIME (with all its pros
and cons).

Jan

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