From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39981) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z3Pdg-00055l-1R for qemu-devel@nongnu.org; Fri, 12 Jun 2015 10:08:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z3PdW-0003VV-MO for qemu-devel@nongnu.org; Fri, 12 Jun 2015 10:08:55 -0400 Received: from lepton.topquark.net ([168.235.66.66]:47841 helo=Mail2.TopQuark.net) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z3PdW-0003VR-Iz for qemu-devel@nongnu.org; Fri, 12 Jun 2015 10:08:46 -0400 Date: Fri, 12 Jun 2015 10:08:45 -0400 From: Paul Donohue Message-ID: <20150612140845.GD2749@TopQuark.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: [Qemu-devel] [PATCH 1/2] qemu-timer: Call clock reset notifiers on forward jumps List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Paolo Bonzini 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 --- 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 e5bd494..0193036 100644 --- a/include/qemu/timer.h +++ b/include/qemu/timer.h @@ -787,6 +787,14 @@ 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 5741f0d..d27be3d 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; -- 2.1.4