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[Qemu-devel] Bug: hpet timer hangs QEMU using icount


From: Xin Li
Subject: [Qemu-devel] Bug: hpet timer hangs QEMU using icount
Date: Tue, 02 Mar 2010 00:06:59 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.7) Gecko/20100120 Fedora/3.0.1-1.fc11 Thunderbird/3.0.1

Hpet hangs QEMU when icount is present. I don't know if this is a universal problem. The cause is that for periodical timers, hpet_timer function tries to insert the timer back to the active timers queue, using an updated expire time. However, before the update, if the expire time is equal to the current time, this updating will be skipped. As a result, this timer will be inserted as is and goes off again and again, basically traps itself in a infinite loop, preventing any new instructions from being executed.

The patch tries to update the timer even when the current time equals the old expire time.

Thanks.

-Xin

--- hw/hpet.c   2010-03-01 23:58:08.815931235 -0500
+++ hw/new_hpet.c       2010-03-01 23:55:07.316704119 -0500
@@ -220,10 +220,10 @@

     if (timer_is_periodic(t) && period != 0) {
         if (t->config & HPET_TN_32BIT) {
-            while (hpet_time_after(cur_tick, t->cmp))
+            while (!hpet_time_after(t->cmp, cur_tick))
                 t->cmp = (uint32_t)(t->cmp + t->period);
         } else
-            while (hpet_time_after64(cur_tick, t->cmp))
+            while (!hpet_time_after64(t->cmp, cur_tick))
                 t->cmp += period;

         diff = hpet_calculate_diff(t, cur_tick);





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