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

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

On 2017/4/19 10:02, Xiao Guangrong wrote:
>
> On 04/13/2017 05:38 PM, Hailiang Zhang wrote:
>> 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.
>>
> Hi Hailiang,
>
> Does this patchset work for you? :)

Yes, i think it works for us, nice work :)

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