[Qemu-devel] 答复: [PATCH 0/5] mc146818rtc: fix Windows VM clock faster

[Zhanghailiang]

Hi,

-----邮件原件-----
发件人: address@hidden [mailto:address@hidden 代表 Xiao Guangrong
发送时间: 2017年4月13日 16:53
收件人: Paolo Bonzini; address@hidden; address@hidden
抄送: address@hidden; address@hidden; address@hidden; 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, address@hidden 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 ?
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
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/


Hailiang