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Re: [Qemu-devel] [PATCH V3] rtc: fix a infinite loop in windows vmstart
From: |
peng.hao2 |
Subject: |
Re: [Qemu-devel] [PATCH V3] rtc: fix a infinite loop in windows vmstartup |
Date: |
Tue, 25 Jul 2017 17:29:02 +0800 (CST) |
>On 25/07/2017 06:14, address@hidden 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