From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=55275 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PkH72-0007VF-0U for qemu-devel@nongnu.org; Tue, 01 Feb 2011 09:21:45 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PkH70-00057z-Ga for qemu-devel@nongnu.org; Tue, 01 Feb 2011 09:21:43 -0500 Received: from thoth.sbs.de ([192.35.17.2]:23212) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PkH70-00057l-6T for qemu-devel@nongnu.org; Tue, 01 Feb 2011 09:21:42 -0500 Message-ID: <4D4816F0.5060009@siemens.com> Date: Tue, 01 Feb 2011 15:21:36 +0100 From: Jan Kiszka MIME-Version: 1.0 References: <20110201124707.GA12061@amt.cnet> <4D480B76.90509@siemens.com> <20110201134821.GA12848@amt.cnet> <4D48116A.9020807@siemens.com> <20110201141039.GA14442@amt.cnet> In-Reply-To: <20110201141039.GA14442@amt.cnet> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: [PATCH 14/22] kvm: Fix race between timer signals and vcpu entry under !IOTHREAD List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Marcelo Tosatti Cc: Stefan Hajnoczi , Avi Kivity , "kvm@vger.kernel.org" , "qemu-devel@nongnu.org" On 2011-02-01 15:10, Marcelo Tosatti wrote: > On Tue, Feb 01, 2011 at 02:58:02PM +0100, Jan Kiszka wrote: >> On 2011-02-01 14:48, Marcelo Tosatti wrote: >>> On Tue, Feb 01, 2011 at 02:32:38PM +0100, Jan Kiszka wrote: >>>> On 2011-02-01 13:47, Marcelo Tosatti wrote: >>>>> On Thu, Jan 27, 2011 at 02:09:58PM +0100, Jan Kiszka wrote: >>>>>> Found by Stefan Hajnoczi: There is a race in kvm_cpu_exec between >>>>>> checking for exit_request on vcpu entry and timer signals arriving >>>>>> before KVM starts to catch them. Plug it by blocking both timer related >>>>>> signals also on !CONFIG_IOTHREAD and process those via signalfd. >>>>>> >>>>>> Signed-off-by: Jan Kiszka >>>>>> CC: Stefan Hajnoczi >>>>>> --- >>>>>> cpus.c | 18 ++++++++++++++++++ >>>>>> 1 files changed, 18 insertions(+), 0 deletions(-) >>>>>> >>>>>> diff --git a/cpus.c b/cpus.c >>>>>> index fc3f222..29b1070 100644 >>>>>> --- a/cpus.c >>>>>> +++ b/cpus.c >>>>>> @@ -254,6 +254,10 @@ static void qemu_kvm_init_cpu_signals(CPUState *env) >>>>>> pthread_sigmask(SIG_BLOCK, NULL, &set); >>>>>> sigdelset(&set, SIG_IPI); >>>>>> sigdelset(&set, SIGBUS); >>>>>> +#ifndef CONFIG_IOTHREAD >>>>>> + sigdelset(&set, SIGIO); >>>>>> + sigdelset(&set, SIGALRM); >>>>>> +#endif >>>>> >>>>> I'd prefer separate qemu_kvm_init_cpu_signals in the !IOTHREAD >>>>> section. >>>> >>>> You mean to duplicate qemu_kvm_init_cpu_signals for both configurations? >>> >>> Yes, so to avoid #ifdefs spread. >> >> Would exchange some #ifdefs against ifndef _WIN32. Haven't measured the >> delta though. >> >>> >>>>>> + >>>>>> +#ifndef CONFIG_IOTHREAD >>>>>> + if (sigismember(&chkset, SIGIO) || sigismember(&chkset, SIGALRM)) { >>>>>> + qemu_notify_event(); >>>>>> + } >>>>>> +#endif >>>>> >>>>> Why is this necessary? >>>>> >>>>> You should break out of cpu_exec_all if there's a pending alarm (see >>>>> qemu_alarm_pending()). >>>> >>>> qemu_alarm_pending() is not true until the signal is actually taken. The >>>> alarm handler sets the required flags. >>> >>> Right. What i mean is you need to execute the signal handler inside >>> cpu_exec_all loop (so that alarm pending is set). >>> >>> So, if there is a SIGALRM pending, qemu_run_timers has highest >>> priority, not vcpu execution. >> >> We leave the vcpu loop (thanks to notify_event), process the signal in >> the event loop and run the timer handler. This pattern is IMO less >> invasive to the existing code, specifically as it is about to die >> long-term anyway. > > You'll probably see poor timer behaviour on smp guests without iothread > enabled. > Still checking, but that would mean the notification mechanism is broken anyway: If IO events do not force us to process them quickly, we already suffer from latencies in SMP mode. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux