From fb1a3a051d89975f26296163066bb0745ecca49d Mon Sep 17 00:00:00 2001 From: Paul Donohue Date: Fri, 12 Jun 2015 10:08:45 -0400 Subject: [PATCH] qemu-timer: Call clock reset notifiers on forward jumps Commit 691a0c9c introduced a mechanism by which QEMU_CLOCK_HOST can notify other parts of the emulator when the host clock has jumped backward. This is used to avoid stalling timers that were scheduled based on the host clock. However, if the host clock jumps forward, then timers that were scheduled based on the host clock may fire rapidly and cause other problems. For example, the mc146818rtc periodic timer will block execution of the VM and consume host CPU while firing every interrupt for the time period that was skipped by the host clock. To correct that problem, this commit fires the reset notification if the host clock jumps forward by more than a hard-coded limit. The limit is currently set to a value of 60 seconds, which should be small enough to prevent excessive timer loops, but large enough to avoid frequent resets in idle VMs. Signed-off-by: Paul Donohue Message-Id: <20150612140845.GD2749@TopQuark.net> Signed-off-by: Paolo Bonzini --- include/qemu/timer.h | 9 +++++++++ qemu-timer.c | 2 +- 2 files changed, 10 insertions(+), 1 deletion(-) diff --git a/include/qemu/timer.h b/include/qemu/timer.h index e5bd494c07..9e4f90f4aa 100644 --- a/include/qemu/timer.h +++ b/include/qemu/timer.h @@ -787,6 +787,15 @@ static inline int64_t get_ticks_per_sec(void) return 1000000000LL; } +static inline int64_t get_max_clock_jump(void) +{ + /* This should be small enough to prevent excessive interrupts from being + * generated by the RTC on clock jumps, but large enough to avoid frequent + * unnecessary resets in idle VMs. + */ + return 60 * get_ticks_per_sec(); +} + /* * Low level clock functions */ diff --git a/qemu-timer.c b/qemu-timer.c index 5741f0d0e0..aa6757e359 100644 --- a/qemu-timer.c +++ b/qemu-timer.c @@ -573,7 +573,7 @@ int64_t qemu_clock_get_ns(QEMUClockType type) now = get_clock_realtime(); last = clock->last; clock->last = now; - if (now < last) { + if (now < last || now > (last + get_max_clock_jump())) { notifier_list_notify(&clock->reset_notifiers, &now); } return now;