Re: [Qemu-devel] [PATCH V3] rtc: fix a infinite loop in windows vmstartup

[<peng.hao2@zte.com.cn>]

>On 25/07/2017 06:14, peng.hao2@zte.com.cn wrote:





>>> On 24/07/2017 20:35, Peng Hao wrote:
>> 
>> 
>> 
>> 
>> 
>>>> When a windows vm starts, periodic timer of rtc will stop several times.
>>>> windows kernel will check whether REG_A_UIP is changed. REG_C's interrupt
>>>> flags will not be cleared when periodic timer stops and the update timer
>>>> will switch to alarm timer. So the expiration time of alarm timer is very
>>>> long and REG_A_UIP will not vary.At last windows kernel will repeat to 
>>>> check REG_A_UIP all the time.
>> 
>>> This should not happen.  REG_A_UIP is set and cleared in register A
>>> every second, like this:
>>>        case RTC_REG_A:
>>>            if (update_in_progress(s)) {
>>>                s->cmos_data[s->cmos_index] |= REG_A_UIP
>>>            } else {
>>>                s->cmos_data[s->cmos_index] &= ~REG_A_UIP
>>>            }
>>>            ret = s->cmos_data[s->cmos_index]
>>>            break
>> 
>> 
>> 
>> when periodic timer stop, update timer is set to a long expire time (as alarm timer).

>I think I see the bug now:

>diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
>index 1b8d3d7d4c..6184b4378e 100644
>--- a/hw/timer/mc146818rtc.c
>+++ b/hw/timer/mc146818rtc.c
>@@ -321,9 +321,11 @@ static void check_update_timer(RTCState *s)
>     s->next_alarm_time = next_update_time +
>                          (next_alarm_sec - 1) * NANOSECONDS_PER_SECOND
> 
>-    if (s->cmos_data[RTC_REG_C] & REG_C_UF) {
>-        /* UF is set, but AF is clear.  Program the timer to target
>-         * the alarm time.  */
>+    if ((s->cmos_data[RTC_REG_C] & REG_C_UF) &&
>+        !(s->cmos_data[RTC_REG_A] & REG_A_UIP)) {
>+        /* If UIP was latched, we need to clear it at the next update.
>+         * Otherwise, if UF is set we only need to program the timer to
>+         * target the alarm time.  */
>         next_update_time = s->next_alarm_time
>     }
>     if (next_update_time != timer_expire_time_ns(s->update_timer)) {
>but I would like to have a testcase for it in tests/rtc-test.c.

>Can you check if the above works and try writing a testcase (that fails 
>without the patch and succeeds with it)?
>Thanks,
I don't  think it can works. REG_C_UF is totally cleared by periodic timer in original code.

after periodic timer stoped, the REG_C_UF  is never cleared.

rtc_update_timer has cleared REG_A_UIP,I think  the patch  does nothing.

I reproduce the bug when many windows VMs reboot and it is about the method of  windows kernel accessing rtc .

so i don't know how to write the testcase.




>Paolo