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

[Dor Laor <dlaor@redhat.com>]

On 09/08/2009 08:00 PM, Jan Kiszka wrote:
> 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".

btw: this is not a hack, its virtualization support for rtc.
It should be the default when qemu runs along with kvm.

>
>>
>> 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
>