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

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

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.

Jan

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