Hi,
We tested with the latest QEMU, and found that time drift obviously (clock fast
in guest)
in Windows 7 64 bits guest in some cases.
It is easily to reproduce, using the follow QEMU command line to start windows
7:
# x86_64-softmmu/qemu-system-x86_64 -name win7_64_2U_raw -machine
pc-i440fx-2.6,accel=kvm,usb=off -cpu host -m 2048 -realtime mlock=off -smp
4,sockets=2,cores=2,threads=1 -rtc base=utc,clock=vm,driftfix=slew -no-hpet
-global kvm-pit.lost_tick_policy=discard -hda /mnt/nfs/win7_sp1_32_2U_raw -vnc
:11 -netdev tap,id=hn0,vhost=off -device rtl8139,id=net-pci0,netdev=hn0 -device
piix3-usb-uhci,id=usb -device usb-tablet,id=input0 -device usb-mouse,id=input1
-device usb-kbd,id=input2 -monitor stdio
Adjust the VM's time to host time, and run java application or run the follow
program
in windows 7:
#pragma comment(lib, "winmm")
#include <stdio.h>
#include <windows.h>
#define SWITCH_PEROID 13
int main()
{
DWORD count = 0;
while (1)
{
count++;
timeBeginPeriod(1);
DWORD start = timeGetTime();
Sleep(40);
timeEndPeriod(1);
if ((count % SWITCH_PEROID) == 0) {
Sleep(1);
}
}
return 0;
}
After few minutes, you will find that the time in windows 7 goes ahead of the
host time, drifts about several seconds.
I have dug deeper in this problem. For windows systems that use the CMOS timer,
the base interrupt rate is usually 64Hz, but running some application in VM
will raise the timer rate to 1024Hz, running java application and or above
program will raise the timer rate.
Besides, Windows operating systems generally keep time by counting timer
interrupts (ticks). But QEMU seems not emulate the rate converting fine.
We update the timer in function periodic_timer_update():
static void periodic_timer_update(RTCState *s, int64_t current_time)
{
cur_clock = muldiv64(current_time, RTC_CLOCK_RATE, get_ticks_per_sec());
next_irq_clock = (cur_clock & ~(period - 1)) + period;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Here we calculate the next interrupt time by align the current clock with the
new period, I'm a little confused that why we care about the *history* time ?
If VM switches from high rate to low rate, the next interrupt time may come
earlier than it supposed to be. We have observed it in our test. we printed the
interval time of interrupts and the VM's current time (We got the time from VM).
Here is part of the log:
... ...
period=512 irq inject 1534: 15625 us
Tue Mar 29 04:38:00 2016
*irq_num_period_32=0, irq_num_period_512=64: [3]: Real time interval is 999696
us
... ...
*irq_num_period_32=893, irq_num_period_512=9 [81]: Real time interval is 951086
us
Convert 32 --- > 512: 703: 96578 us
period=512 irq inject 44391: 12702 us
Convert 512 --- > 32: 704: 12704 us11
period=32 irq inject 44392: 979 us
... ...
32 --- > 512: 705: 24388 us
period=512 irq inject 44417: 6834 us
Convert 512 --- > 32: 706: 6830 us
period=32 irq inject 44418: 978 us
... ...
Convert 32 --- > 512: 707: 60525 us
period=512 irq inject 44480: 1945 us
Convert 512 --- > 32: 708: 1955 us
period=32 irq inject 44481: 977 us
... ...
Convert 32 --- > 512: 709: 36105 us
period=512 irq inject 44518: 10741 us
Convert 512 --- > 32: 710: 10736 us
period=32 irq inject 44519: 989 us
... ...
Convert 32 --- > 512: 711: 123998 us
period=512 irq inject 44646: 974 us
period=512 irq inject 44647: 15607 us
Convert 512 --- > 32: 712: 16560 us
period=32 irq inject 44648: 980 us
... ...
period=32 irq inject 44738: 974 us
Convert 32 --- > 512: 713: 88828 us
period=512 irq inject 44739: 4885 us
Convert 512 --- > 32: 714: 4882 us
period=32 irq inject 44740: 989 us
... ...
period=32 irq inject 44842: 974 us
Convert 32 --- > 512: 715: 100537 us
period=512 irq inject 44843: 8788 us
Convert 512 --- > 32: 716: 8789 us
period=32 irq inject 44844: 972 us
... ...
period=32 irq inject 44941: 979 us
Convert 32 --- > 512: 717: 95677 us
period=512 irq inject 44942: 13661 us
Convert 512 --- > 32: 718: 13657 us
period=32 irq inject 44943: 987 us
... ...
Convert 32 --- > 512: 719: 94690 us
period=512 irq inject 45040: 14643 us
Convert 512 --- > 32: 720: 14642 us
period=32 irq inject 45041: 974 us
... ...
Convert 32 --- > 512: 721: 88848 us
period=512 irq inject 45132: 4892 us
Convert 512 --- > 32: 722: 4931 us
period=32 irq inject 45133: 964 us
... ...
Tue Mar 29 04:39:19 2016
*irq_num_period_32:835, irq_num_period_512:11 [82], Real time interval is
911520 us
For windows 7, it has got 835 IRQs which injected during the period of 32,
and got 11 IRQs that injected during the period of 512. it updated the
wall-clock
time with one second, because it supposed it has counted
(835*976.5+11*15625)= 987252.5 us, but the real interval time is 911520 us.
IMHO, we should calculate the next interrupt time based on the time of last
interrupt injected, and it seems to be more similar with hardware CMOS timer
in this way.
Maybe someone can tell me the reason why we calculated the interrupt timer
in that way, or is it a bug ? ;)
Thanks,
Hailiang
