From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:42018)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1Uyi9h-0002NC-A5
	for qemu-devel@nongnu.org; Mon, 15 Jul 2013 08:45:34 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1Uyi9g-000123-6Q
	for qemu-devel@nongnu.org; Mon, 15 Jul 2013 08:45:29 -0400
Received: from mx1.redhat.com ([209.132.183.28]:23428)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1Uyi9f-000112-TL
	for qemu-devel@nongnu.org; Mon, 15 Jul 2013 08:45:28 -0400
Message-ID: <51E3EEDA.4080501@redhat.com>
Date: Mon, 15 Jul 2013 14:45:14 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1373027986-17868-1-git-send-email-stefanha@redhat.com>
	<51D7079A.2000907@siemens.com>
In-Reply-To: <51D7079A.2000907@siemens.com>
Content-Type: text/plain; charset=ISO-8859-15
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH 0/3] qemu-timer: make QEMUTimer functions
	thread-safe
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: Jan Kiszka <jan.kiszka@siemens.com>
Cc: alex@alex.org.uk, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, rth@twiddle.net

Il 05/07/2013 19:51, Jan Kiszka ha scritto:
> On 2013-07-05 14:39, Stefan Hajnoczi wrote:
>> This series makes the following functions thread-safe:
>>
>>   qemu_mod_timer_ns()
>>   qemu_mod_timer()
>>   qemu_del_timer()
>>   qemu_timer_pending()
>>
>> The following were already thread-safe:
>>
>>   qemu_free_timer()
>>   qemu_new_timer()
>>   qemu_timer_expired()
>>
>> Now it is possible to use QEMUTimer outside the QEMU global mutex.  Timer
>> callbacks are still invoked from the main loop.  If a thread wishes to run
>> timer callbacks it must use a thread-safe QEMUBH (which Ping Fan Liu is working
>> on).
> 
> What do you mean with this? We need main-loop independent timers for any
> task that depends on timely alarm delivery. Do your patches keep this in
> mind as well?

These are orthogonal issues.  Stefan's usecase does not need timely
delivery.

>> Note that host_clock is not thread-safe because it keeps state and invokes
>> reset notifiers.  Device emulation threads mostly care about vm_clock, so this
>> is not a problem.
> 
> I suppose you know that vm_clock cannot be read outside BQL yet due to
> timers_state and, under TCG, icount. Any ideas regarding this already? I
> didn't have to solve that problem so far as I only need CLOCK_REALTIME
> outside BQL.

I was thinking of a seqlock.  It should be quite cheap, since there
would be hardly any contention.

No ideas about host_clock's notifiers.

Paolo