On 04/13/2017 02:37 PM, Paolo Bonzini wrote:
On 12/04/2017 17:51, [email protected] 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[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...
The series also does more than this fix (or workaround), so I will review it anyway.
Thank you, Paolo!
