From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41856) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7RjL-0006f2-I9 for qemu-devel@nongnu.org; Tue, 23 Jun 2015 13:11:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z7RjI-0004hv-PR for qemu-devel@nongnu.org; Tue, 23 Jun 2015 13:11:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60837) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7RjI-0004hp-A6 for qemu-devel@nongnu.org; Tue, 23 Jun 2015 13:11:24 -0400 Date: Tue, 23 Jun 2015 14:11:22 -0300 From: Eduardo Habkost Message-ID: <20150623171122.GI3134@thinpad.lan.raisama.net> References: <20150608201835.GM3525@orkuz.home> <558951C0.3050806@suse.de> <20150623150828.GD3134@thinpad.lan.raisama.net> <20150623173048-mutt-send-email-mst@redhat.com> <20150623155832.GE3134@thinpad.lan.raisama.net> <55898637.6080804@suse.de> <20150623162555.GL30318@redhat.com> <20150623183115-mutt-send-email-mst@redhat.com> <20150623164204.GM30318@redhat.com> <55898D94.4070702@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <55898D94.4070702@suse.de> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 0/2] target-i386: "custom" CPU model + script to dump existing CPU models List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Andreas =?iso-8859-1?Q?F=E4rber?= Cc: mimu@linux.vnet.ibm.com, "Michael S. Tsirkin" , qemu-devel@nongnu.org, Alexander Graf , borntraeger@de.ibm.com, Igor Mammedov , Paolo Bonzini , Jiri Denemark , rth@twiddle.net On Tue, Jun 23, 2015 at 06:47:16PM +0200, Andreas F=E4rber wrote: > Am 23.06.2015 um 18:42 schrieb Daniel P. Berrange: > > On Tue, Jun 23, 2015 at 06:33:05PM +0200, Michael S. Tsirkin wrote: > >> On Tue, Jun 23, 2015 at 05:25:55PM +0100, Daniel P. Berrange wrote: > >>> On Tue, Jun 23, 2015 at 06:15:51PM +0200, Andreas F=E4rber wrote: > >>>> Am 23.06.2015 um 17:58 schrieb Eduardo Habkost: > >>>>> On Tue, Jun 23, 2015 at 05:32:42PM +0200, Michael S. Tsirkin wrot= e: > >>>>>> On Tue, Jun 23, 2015 at 12:08:28PM -0300, Eduardo Habkost wrote: > >>>>>>> On Tue, Jun 23, 2015 at 02:32:00PM +0200, Andreas F=E4rber wrot= e: > >>>>>>>> Am 08.06.2015 um 22:18 schrieb Jiri Denemark: > >>>>>>>>>> To help libvirt in the transition, a x86-cpu-model-dump scri= pt is provided, > >>>>>>>>>> that will generate a config file that can be loaded using -r= eadconfig, based on > >>>>>>>>>> the -cpu and -machine options provided in the command-line. > >>>>>>>>> > >>>>>>>>> Thanks Eduardo, I never was a big fan of moving (or copying) = all the CPU > >>>>>>>>> configuration data to libvirt, but now I think it actually ma= kes sense. > >>>>>>>>> We already have a partial copy of CPU model definitions in li= bvirt > >>>>>>>>> anyway, but as QEMU changes some CPU models in some machine t= ypes (and > >>>>>>>>> libvirt does not do that) we have no real control over the gu= est CPU > >>>>>>>>> configuration. While what we really want is full control to e= nforce > >>>>>>>>> stable guest ABI. > >>>>>>>> > >>>>>>>> That sounds like FUD to me. Any concrete data points where QEM= U does not > >>>>>>>> have a stable ABI for x86 CPUs? That's what we have the pc*-x.= y machines > >>>>>>>> for. > >>>>>>> > >>>>>>> What Jiri is saying that the CPUs change depending on -mmachine= , not > >>>>>>> that the ABI is broken by a given machine. > >>>>>>> > >>>>>>> The problem here is that libvirt needs to provide CPU models wh= ose > >>>>>>> runnability does not depend on the machine-type. If users have = a VM that > >>>>>>> is running in a host and the VM machine-type changes, > >>>>>> > >>>>>> How does it change, and why? > >>>>> > >>>>> Sometimes we add features to a CPU model because they were not em= ulated by KVM > >>>>> and now they are. Sometimes we remove or add features or change o= ther fields > >>>>> because we are fixing previous mistakes. Recently we we were goin= g to remove > >>>>> features from models because of an Intel CPU errata, but then dec= ided to create > >>>>> a new CPU model name instead. > >>>>> > >>>>> See some examples at the end of this message. > >>>>> > >>>>>> > >>>>>>> the VM should be > >>>>>>> still runnable in that host. QEMU doesn't provide that, our CPU= models > >>>>>>> may change when we introduce new machine-types, so we are givin= g them a > >>>>>>> mechanism that allows libvirt to implement the policy they need= . > >>>>>> > >>>>>> I don't mind wrt CPU specifically, but we absolutely do change g= uest ABI > >>>>>> in many ways when we change machine types. > >>>>> > >>>>> All the other ABI changes we introduce in QEMU don't affect runna= bility of the > >>>>> VM in a given host, that's the problem we are trying to address h= ere. ABI > >>>>> changes are expected when changing to a new machine, runnability = changes > >>>>> aren't. > >>>>> > >>>>> > >>>>> Examples of commits changing CPU models: > >>>> [snip] > >>>> > >>>> I've always advocated remaining backwards-compatible and only maki= ng CPU > >>>> model changes for new machines. You among others felt that was not > >>>> always necessary, and now you're using the lack thereof as an argu= ment > >>>> to stop using QEMU's CPU models at all? That sounds convoluted... > >>> > >>> Whether QEMU changed the CPU for existing machines, or only for new > >>> machines is actually not the core problem. Even if we only changed > >>> the CPU in new machines that would still be an unsatisfactory situa= tion > >>> because we want to be able to be able to access different versions = of > >>> the CPU without the machine type changing, and access different ver= sions > >>> of the machine type, without the CPU changing. IOW it is the fact t= hat the > >>> changes in CPU are tied to changes in machine type that is the core > >>> problem. > >> > >> But that's because we are fixing bugs. If CPU X used to work on > >> hardware Y in machine type A and stopped in machine type B, this is > >> because we have determined that it's the right thing to do for the > >> guests and the users. We don't break stuff just for fun. > >> Why do you want to bring back the bugs we fixed? > >=20 > > Huh, I never said we wanted to bring back bugs. This is about allowin= g > > libvirt to fix the CPU bugs in a way that is independant of the machi= ne > > types and portable across hypervisors we deal with. We're absolutely > > still going to fix CPU model bugs and ensure stable guest ABI. >=20 > No, that's contradictory! Through the -x.y machines we leave bugs in th= e > old models *exactly* to assure a stable guest ABI. Fixes are only be > applied to new machines, thus I'm pointing out that you should not use = a > new CPU model with an old machine type. They don't need to use a new model with an old machine-type (although I don't see a reason to prevent that). They need to be able to change the machine-type to a new one without getting any changes that would make the VM not runnable in the same host. Even if it is a bug fix. If it is a change that can make the VM unrunnable, it needs to be controlled by a separate flag, not by the machine-type. --=20 Eduardo