From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:45495) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RBUTm-0006lm-4L for qemu-devel@nongnu.org; Wed, 05 Oct 2011 12:37:59 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1RBUTk-0006hm-TX for qemu-devel@nongnu.org; Wed, 05 Oct 2011 12:37:58 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58838) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RBUTk-0006hh-LG for qemu-devel@nongnu.org; Wed, 05 Oct 2011 12:37:56 -0400 Message-ID: <4E8C87DF.5080201@redhat.com> Date: Wed, 05 Oct 2011 18:37:51 +0200 From: Avi Kivity MIME-Version: 1.0 References: <1317729885-17534-1-git-send-email-pbonzini@redhat.com> <20111005113707.5312f98b@doriath> <4E8C7B1C.2020808@redhat.com> <4E8C8656.3040706@web.de> In-Reply-To: <4E8C8656.3040706@web.de> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] runstate: do not discard runstate changes when paused List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Paolo Bonzini , qemu-devel@nongnu.org, Luiz Capitulino On 10/05/2011 06:31 PM, Jan Kiszka wrote: > >> > > > > vm_start() should be symmetric with vm_stop(). That is, if a piece of > > code wants to execute with vcpus stopped, it should just run inside a > > stop/start pair. > > > > The only confusion can come from the user, if he sees multiple stop > > events and expects that just one cont will continue the vm. For the > > machine monitor, we should just document that the you have to issue one > > cont for every stop event you see (plus any stops you issue). It's not > > unnatural - the code that handles a stop_due_to_enospace can work to fix > > the error and issue a cont, disregarding any other stops in progress > > (due to a user pressing the stop button, or migration, or cpu hotplug, > > or whatever). For the human monitor, it's not so intuitive, but the > > situation is so rare we can just rely on the user to issue cont again. > > Making this kind of user-visible change would be a bad idea. The current situation is a bad idea. Consider a user-initiated or qemu-initiated stop; the user starts to deal with it, types 'cont', and as the Enter key is being depressed another qemu-initiated stop comes along. The 'cont' restarts the guest even though the second event was not dealt with. > We are talking about multiple stop states here, but only a single > function (vm_stop) to enter them - maybe that's not optimal. But the > point is that we were missing one stop-to-stop transition. And that > needs to be fixed, either inside vm_stop or when it is called. Those stops are orthogonal. There is no relationship between a migration stop, a user stop, an ENOSPC stop, a hotplug stop, and a debugger stop. There is no reason to start inventing stop-to-stop transitions between them. A 'cont' intended for one should not undo another. There are two ways to do this, one is to store a set of stop reasons and let both 'stop' and 'cont' specify the reason, the other, which is simpler but less safe, is to use a reference counting approach. > > If you want to lock the VM into paused state, add a new symmetric API > that does precisely this. That API would send the VM into RSTATE_LOCKED > if it is not yet stopped on lock or is still locked on resume. That > would avoid redefining stop states that have no use for lock-counting > semantics. > Which stop states would these be? When would you want one cont to undo two stops? -- error compiling committee.c: too many arguments to function