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Re: [Qemu-devel] [PATCH 2/2] [RFC] time: refactor QEMU timer to use GHRT
From: |
Jan Kiszka |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] [RFC] time: refactor QEMU timer to use GHRTimer |
Date: |
Tue, 23 Aug 2011 01:48:47 +0200 |
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Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 |
On 2011-08-22 23:55, Anthony Liguori wrote:
>>>> These two assessments are partly just wrong, partly fail to see the
>>>> real
>>>> use case. QEMU_CLOCK_HOST serves the very valid scenarios where a guest
>>>> clock shall be kept synchronized on the host time, also following its
>>>> jumps accordingly without stalling timers.
>>>
>>> The only reason we see jumps at all is because we're using
>>> CLOCK_MONOTONIC or CLOCK_REALTIME. If we used CLOCK_MONOTONIC_RAW, we
>>> don't see any jumps at all.
>>
>> CLOCK_MONOTONIC will not jump backward as well, so is perfectly fine and
>> better portable. Backward jumps cannot be avoided when using a host
>> system clock that is subject to follow a more accurate external source.
>> But having such source for RTC emulation e.g. is a useful feature.
>
> I think its of limited utility. The RTC isn't universally used for time
> keeping. There's also no guarantee that the guest isn't going to be
> upset by this.
That's why it's configurable. On the other hand, most guests are
well-prepared to deal with the RTC and the host runtime clock source not
being in sync. That's why we had no bug reports after making this mode
default.
>
> I think a better approach is to simply have a verb in qemu-ga to set/get
> the guest time. That let's you implement clock adjustment without
> having to worry about NTP. I'm happy to add that as part of this series.
That's not that easy in legacy guests, even more when they aren't Linux.
>
> I don't think messing around with this stuff belongs in the QEMU clock
> layer though.
It's really not messing, the impact is almost minimal.
Let's focus on the real big pieces first and then see what CLOCK_HOST
actually contributes. That also means you should split up you patches a
bit more, already for better bisectability.
Jan
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Re: [Qemu-devel] [PATCH 2/2] [RFC] time: refactor QEMU timer to use GHRTimer, Paolo Bonzini, 2011/08/23
Re: [Qemu-devel] [PATCH 1/2] main: add high resolution GSource based timer, Anthony Liguori, 2011/08/22