Re: 答复: [PATCH 0/5] mc146818rtc: fix Windows VM clock faster

[Hailiang Zhang <zhang.zhanghailiang@huawei.com>]

On 2017/4/13 17:35, Xiao Guangrong wrote:
>
> On 04/13/2017 05:29 PM, Hailiang Zhang wrote:
>> On 2017/4/13 17:18, Xiao Guangrong wrote:
>>> On 04/13/2017 05:05 PM, Zhanghailiang wrote:
>>>> Hi,
>>>>
>>>> -----邮件原件-----
>>>> 发件人: kvm-owner@vger.kernel.org [mailto:kvm-owner@vger.kernel.org]
>>>> 代表 Xiao Guangrong
>>>> 发送时间: 2017年4月13日 16:53
>>>> 收件人: Paolo Bonzini; mst@redhat.com; mtosatti@redhat.com
>>>> 抄送: qemu-devel@nongnu.org; kvm@vger.kernel.org;
>>>> yunfangtai@tencent.com; Xiao Guangrong
>>>> 主题: Re: [PATCH 0/5] mc146818rtc: fix Windows VM clock faster
>>>>
>>>>
>>>>
>>>> On 04/13/2017 04:39 PM, Xiao Guangrong wrote:
>>>>> On 04/13/2017 02:37 PM, Paolo Bonzini wrote:
>>>>>> On 12/04/2017 17:51, guangrong.xiao@gmail.com wrote:
>>>>>>> The root cause is that the clock will be lost if the periodic period
>>>>>>> is changed as currently code counts the next periodic time like this:
>>>>>>>         next_irq_clock = (cur_clock & ~(period - 1)) + period;
>>>>>>>
>>>>>>> consider the case if cur_clock = 0x11FF and period = 0x100, then the
>>>>>>> next_irq_clock is 0x1200, however, there is only 1 clock left to
>>>>>>> trigger the next irq. Unfortunately, Windows guests (at least
>>>>>>> Windows7) change the period very frequently if it runs the attached
>>>>>>> code, so that the lost clock is accumulated, the wall-time become
>>>>>>> faster and faster
>>>>>> Very interesting.
>>>>>>
>>>>> Yes, indeed.
>>>>>
>>>>>> However, I think that the above should be exactly how the RTC should
>>>>>> work.  The original RTC circuit had 22 divider stages (see page 13 of
>>>>>> the datasheet[1], at the bottom right), and the periodic interrupt
>>>>>> taps the rising edge of one of the dividers (page 16, second
>>>>>> paragraph).  The datasheet also never mentions a comparator being
>>>>>> used to trigger the periodic interrupts.
>>>>>>
>>>>> That was my thought before, however, after more test, i am not sure if
>>>>> re-configuring RegA changes these divider stages internal...
>>>>>
>>>>>> Have you checked that this Windows bug doesn't happen on real
>>>>>> hardware too?  Or is the combination of driftfix=slew and changing
>>>>>> periods that is a problem?
>>>>>>
>>>>> I have two physical windows 7 machines, both of them have
>>>>> 'useplatformclock = off' and ntp disabled, the wall time is really
>>>>> accurate. The difference is that the physical machines are using Intel
>>>>> Q87 LPC chipset which is mc146818rtc compatible. However, on VM, the
>>>>> issue is easily be reproduced just in ~10 mins.
>>>>>
>>>>> Our test mostly focus on 'driftfix=slew' and after this patchset the
>>>>> time is accurate and stable.
>>>>>
>>>>> I will do the test for dropping 'slew' and see what will happen...
>>>>>
>>>>> Well, the time is easily observed to be faster if 'driftfix=slew' is
>>>>> not used. :(
>>>> You mean, it only fixes the one case which with the ' driftfix=slew '
>>>> is used ?
>>> No. for both.
>>>
>>>> We encountered this problem too, I have tried to fix it long time ago.
>>>> https://lists.gnu.org/archive/html/qemu-devel/2016-03/msg06937.html.
>>>> (It seems that your solution is more useful)
>>>> But it seems that it is impossible to fix, we need to emulate the
>>>> behaviors of real hardware,
>>>> but we didn't find any clear description about it. And it seems that
>>>> other virtualization platforms
>>> That is the issue, the hardware spec does not detail how the clock is
>>> counted when the timer interval is changed. What we can do at this time
>>> is that speculate it from the behaviors. Current RTC is completely
>>> unusable anyway.
>>>
>>>
>>>> have this problem too:
>>>> VMware:
>>>> https://www.vmware.com/files/pdf/Timekeeping-In-VirtualMachines.pdf
>>>> Heper-v:
>>>> https://blogs.msdn.microsoft.com/virtual_pc_guy/2010/11/19/time-synchronization-in-hyper-v/
>>>>
>>> Hmm, slower clock is understandable, does really the Windows7 on hyperV
>>> have faster clock? Did you meet it?
>> I don't know, we didn't test it, besides, I'd like to know how long did
>> your testcase run before
>> you judge it is stable with 'driftfix=slew'  option? (My previous patch
>> can't fix it completely but
>> only narrows the gap between timer in guest and real timer.)
> More than 12 hours.

Great, I'll test and look into it ... thanks.

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