From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56408) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7RE1-0005IM-68 for qemu-devel@nongnu.org; Tue, 23 Jun 2015 12:39:06 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z7RDw-0005MM-OC for qemu-devel@nongnu.org; Tue, 23 Jun 2015 12:39:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38331) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7RDw-0005ME-GA for qemu-devel@nongnu.org; Tue, 23 Jun 2015 12:39:00 -0400 Date: Tue, 23 Jun 2015 13:38:58 -0300 From: Eduardo Habkost Message-ID: <20150623163858.GG3134@thinpad.lan.raisama.net> References: <1433790460-30679-1-git-send-email-ehabkost@redhat.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20150623183115-mutt-send-email-mst@redhat.com> 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: "Michael S. Tsirkin" Cc: mimu@linux.vnet.ibm.com, qemu-devel@nongnu.org, Alexander Graf , borntraeger@de.ibm.com, Igor Mammedov , Paolo Bonzini , Jiri Denemark , Andreas =?iso-8859-1?Q?F=E4rber?= , rth@twiddle.net 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? > > > >=20 > > > > 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. > > > >=20 > > > > See some examples at the end of this message. > > > >=20 > > > >> > > > >>> 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. > > > >=20 > > > > 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. > > > >=20 > > > >=20 > > > > Examples of commits changing CPU models: > > > [snip] > > >=20 > > > I've always advocated remaining backwards-compatible and only makin= g 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 argum= ent > > > to stop using QEMU's CPU models at all? That sounds convoluted... > >=20 > > 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 situati= on > > because we want to be able to be able to access different versions of > > the CPU without the machine type changing, and access different versi= ons > > of the machine type, without the CPU changing. IOW it is the fact tha= t the > > changes in CPU are tied to changes in machine type that is the core > > problem. > >=20 > > Regards, > > Daniel >=20 > 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? I didn't take the time to count them, but I bet most of the commits I listed on my previous e-mail message are not bug fixes, but new features. Also, it doesn't matter if the change is a bug fix or a new feature: if it affects runnability of the VM, it has more consequences than a simple guest-side ABI change, and libvirt can't tie it to the machine-type so it needs another flag to enable it. --=20 Eduardo