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Re: [Qemu-devel] [PATCH V3] rtc: fix a infinite loop in windows vm start
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH V3] rtc: fix a infinite loop in windows vm startup |
Date: |
Tue, 25 Jul 2017 09:17:44 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 |
On 25/07/2017 06:14, address@hidden wrote:
>> On 24/07/2017 20:35, Peng Hao wrote:
>
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>>> 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,
Paolo