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

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

Jan Kiszka wrote:
> 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

+ ,td-hack=on|off

of course. Or let's call it "drift-hack".

> 
> 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
> 

Jan

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