From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:51904)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1YB8p1-0001Gx-2x
	for qemu-devel@nongnu.org; Tue, 13 Jan 2015 16:16:20 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1YB8ox-0004eX-LL
	for qemu-devel@nongnu.org; Tue, 13 Jan 2015 16:16:19 -0500
Received: from mx1.redhat.com ([209.132.183.28]:41834)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1YB8ox-0004eQ-Ds
	for qemu-devel@nongnu.org; Tue, 13 Jan 2015 16:16:15 -0500
Message-ID: <54B58B18.9060205@redhat.com>
Date: Tue, 13 Jan 2015 22:16:08 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <54AE87C1.2060907@wiesinger.com>	<54AEBD43.2060705@redhat.com>	<54AEC877.9080600@wiesinger.com>	<54AECAF3.3060909@redhat.com>	<54AF047D.8010009@wiesinger.com>
	<54B3B2F5.1090405@wiesinger.com>	<54B57C51.7090002@wiesinger.com>
	<54B584AB.4090303@redhat.com> <54B58AC0.5080805@wiesinger.com>
In-Reply-To: <54B58AC0.5080805@wiesinger.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] Fedora FC21 - Bug: 100% CPU and hangs in
 gettimeofday(&tp, NULL); forever
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Gerhard Wiesinger <lists@wiesinger.com>, qemu-devel@nongnu.org, Cole Robinson <crobinso@redhat.com>, virt@lists.fedoraproject.org



On 13/01/2015 22:14, Gerhard Wiesinger wrote:
> 
> I also had a look at the kernel code again:
> http://lxr.free-electrons.com/source/kernel/time/timekeeping.c?v=3.17#L493
> 499         do {
> 500                 seq = read_seqcount_begin(&tk_core.seq);
> 501
> 502                 ts->tv_sec = tk->xtime_sec;
> 503                 nsecs = timekeeping_get_ns(&tk->tkr);
> 504
> 505         } while (read_seqcount_retry(&tk_core.seq, seq));
> 
> So it looks like that the seqcount always changes and therefore loops
> forever here (as far as I digged it down this is the only loop here).
> 
> Might be something wrong with the memory barriers in recent qemu-kvm
> releases?

No, that's not possible.  Unless you pause/resume or migrate the VM, all
of the handling of kvmclock is entirely in the kernel.

Paolo