From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:43399) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V3qXJ-0002kw-Up for qemu-devel@nongnu.org; Mon, 29 Jul 2013 12:43:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1V3qXF-0000ns-4p for qemu-devel@nongnu.org; Mon, 29 Jul 2013 12:43:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:61719) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V3qXE-0000nO-TT for qemu-devel@nongnu.org; Mon, 29 Jul 2013 12:43:01 -0400 Message-ID: <51F69B7C.7060201@redhat.com> Date: Mon, 29 Jul 2013 18:42:36 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1374159757-16383-1-git-send-email-fred.konrad@greensocs.com> <51E80B3B.6050604@redhat.com> <51F689E0.9050804@greensocs.com> In-Reply-To: <51F689E0.9050804@greensocs.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC 0/3] Determinitic behaviour with icount. List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Frederic Konrad Cc: Peter Maydell , mark.burton@greensocs.com, qemu-devel@nongnu.org Il 29/07/2013 17:27, Frederic Konrad ha scritto: > On 18/07/2013 17:35, Paolo Bonzini wrote: >> Il 18/07/2013 17:06, Peter Maydell ha scritto: >>> On 18 July 2013 16:02, wrote: >>>> As I said in the last email, we have issues with determinism with >>>> icount. >>>> We are wondering if determinism is really ensured with icount? >>> My opinion is that it *should* be deterministic but it would >>> be unsurprising if the determinism had got broken along the way. >> First of all, it can only be deterministic if the guest satisfies (at >> least) all the following condition: >> >> 1) only uses timer that QEMU bases on vm_clock (which means that you >> should use "-rtc clock=vm"---sorry Fred, didn't think about this in the >> previous answer); >> >> 2) never does any network operation nor any asynchronous disk I/O >> operation >> >> 3) never halts the VCPU waiting for an interrupt > > Hi, > > qemu_alarm is making the replay not deterministic too. What is qemu_alarm? If you mean qemu_alarm_timer, then that means rt_clock and host_clock (item 1 above)? If so, yes, I believe you need to record/replay them. When doing replay for reverse execution, you certainly want to execute at full speed without waiting for real time to pass again. Paolo > We tried to remove those alarms and it seems to replay well (at least > far better). > > So the question is: how we can solve that? > > We thought at two possibilities : > * record/replay them, like IO. > * base them on our new ic_clock. > > Both have drawbacks: > * record/replay won't make icount more deterministic (run to run). > * ic_clock speed time is apparently not constant. > > Thanks, > Fred >