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From: | Dor Laor |
Subject: | Re: [Qemu-devel] Re: [PATCH] re-set rtc date on reset handler |
Date: | Wed, 09 Sep 2009 15:28:23 +0300 |
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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<address@hidden> 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). JanJan
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