From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39949) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d6GPO-0007sA-UW for qemu-devel@nongnu.org; Thu, 04 May 2017 09:03:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1d6GPK-0001Ow-2s for qemu-devel@nongnu.org; Thu, 04 May 2017 09:03:03 -0400 Received: from mail.ispras.ru ([83.149.199.45]:40948) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d6GPJ-0001Od-RF for qemu-devel@nongnu.org; Thu, 04 May 2017 09:02:58 -0400 From: "Pavel Dovgalyuk" References: <20170504084135.7488.24715.stgit@PASHA-ISP> <20170504084210.7488.37561.stgit@PASHA-ISP> <000401d2c4c7$6d5b6d50$481247f0$@ru> <000801d2c4cd$3d3af720$b7b0e560$@ru> <58398574-88b5-3382-65e2-7208beccc09a@redhat.com> <000f01d2c4cf$7479f630$5d6de290$@ru> <001101d2c4d2$bb44a4e0$31cdeea0$@ru> In-Reply-To: Date: Thu, 4 May 2017 16:02:59 +0300 Message-ID: <001201d2c4d6$c26bdd20$47439760$@ru> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Language: ru Subject: Re: [Qemu-devel] [PATCH v9 06/10] replay: fix save/load vm for non-empty queue List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: 'Paolo Bonzini' , 'Pavel Dovgalyuk' , qemu-devel@nongnu.org Cc: kwolf@redhat.com, peter.maydell@linaro.org, mst@redhat.com, jasowang@redhat.com, quintela@redhat.com, kraxel@redhat.com > From: Paolo Bonzini [mailto:pbonzini@redhat.com] > On 04/05/2017 14:34, Pavel Dovgalyuk wrote: > >> From: Paolo Bonzini [mailto:pbonzini@redhat.com] > >> On 04/05/2017 14:10, Pavel Dovgalyuk wrote: > >>>> From: Paolo Bonzini [mailto:pbonzini@redhat.com] > >>>> On 04/05/2017 13:54, Pavel Dovgalyuk wrote: > >>>>>> From: Paolo Bonzini [mailto:pbonzini@redhat.com] > >>>>>> On 04/05/2017 13:13, Pavel Dovgalyuk wrote: > >>>>>>>>> This patch does not allows saving/loading vmstate when > >>>>>>>>> replay events queue is not empty. There is no reliable > >>>>>>>>> way to save events queue, because it describes internal > >>>>>>>>> coroutine state. Therefore saving and loading operations > >>>>>>>>> should be deferred to another record/replay step. > >>>>>>>> > >>>>>>>> Can it actually be non-empty after bdrv_drain_all? > >>>>>>> > >>>>>>> drain/flush cannot succeed, because started requests are > >>>>>>> prisoned in the replay events queue. > >>>>>> > >>>>>> But that would apply to loading only. Saving should still be always > >>>>>> possible. > >>>>> > >>>>> We can save it. But it wouldn't load correctly - replay queue will be empty after > loading. > >>>> > >>>> When saving you can drain, and then the events queue should be empty. > >>>> Or I am misunderstanding how it works, which is possible too. > >>> > >>> Drain will wait until the queue becomes empty. > >>> Queue is processed only at checkpoints. > >>> Checkpoints are met in iothread (at timers processing and so on). > >>> But iothread is waiting for finishing the drain. > >> > >> What checkpoint can hang the drain during a save? > > > > No one. There are no checkpoins during vmsave and during drain. > > Therefore the queue will never become empty. > > Understood. And what checkpoint will you be waiting for during drain, > causing a deadlock? Every checkpoint processes the queue, but none of them are invoked, because iothread (which invokes all the checkpoints) is waiting for the end of the drain. And we cannot add a checkpoint into the drain, because then vmstop will affect the behavior of the guest. Pavel Dovgalyuk