|Subject:||Re: [Qemu-devel] 答复: [PATCH 0/5] mc146818rtc: fix Windows VM clock faster|
|Date:||Thu, 13 Apr 2017 17:29:54 +0800|
|User-agent:||Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1|
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: 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 fasterVery 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, 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 platformsThat 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.) Hailiang
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