(CCing libvirt people) On Tue, May 28, 2013 at 06:48:52PM +0200, Andreas Färber wrote: > Am 28.05.2013 18:46, schrieb Paolo Bonzini: > > Il 28/05/2013 18:34, Bandan Das ha scritto: > >> Eduardo Habkost <ehabkost@xxxxxxxxxx> writes: > >> > >>> On Mon, May 27, 2013 at 02:21:36PM +0200, Paolo Bonzini wrote: > >>>> Il 27/05/2013 14:09, Eduardo Habkost ha scritto: > >>>>> On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote: > >>>>>> Il 25/05/2013 03:21, Bandan Das ha scritto: > >>>>>>> There is one user-visible effect: "-cpu ...,enforce" will stop failing > >>>>>>> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly > >>>>>>> the point: there's no point in having CPU model definitions that would > >>>>>>> never work as-is with neither TCG or KVM. This patch is changing the > >>>>>>> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what > >>>>>>> was already happening in practice. > >>>>>> > >>>>>> But then -cpu Opteron_G3 does not match a "real" Opteron G3. Is it > >>>>>> worth it? > >>>>> > >>>>> No models match a "real" CPU this way, because neither TCG or KVM > >>>>> support all features supported by a real CPU. I ask the opposite > >>>>> question: is it worth maintaining an "accurate" CPU model definition > >>>>> that would never work without feature-bit tweaking in the command-line? > >>>> > >>>> It would work with TCG. Changing TCG to KVM should not change hardware > >>>> if you use "-cpu ...,enforce", so it is right that it fails when > >>>> starting with KVM. > >>>> > >>> > >>> Changing between KVM and TCG _does_ change hardware, today (with or > >>> without check/enforce). All CPU models on TCG have features not > >>> supported by TCG automatically removed. See the "if (!kvm_enabled())" > >>> block at x86_cpu_realizefn(). > >> > >> Yes, this is exactly why I was inclined to remove the monitor flag. > >> We already have uses of kvm_enabled() to set (or remove) kvm specific stuff, > >> and this change is no different. > > > > Do any of these affect something that is part of x86_def_t? > > The vendor comes to mind. I believe we can still consider the "vendor" field a special one: if other components care about the TCG/KVM difference regarding the "vendor" field, they can simply set "vendor" explicitly on the command-line. > >> I can see Paolo's point though, having > >> a common definition probably makes sense too. > > Paolo is convincing me that keeping the rest of the features exactly the same on TCG and KVM modes (and making check/enforce work for TCG as well) would simplify the logic a lot. This will add a little extra work for libvirt, that will probably need to use "-cpu Opteron_G3,-monitor" once it implements enforce-mode (to make sure the results really match existing libvirt assumptions about the Opteron_G* models), but it is probably worth it. I will give it a try and send a proposal soon. > >>> (That's why I argue that we need separate classes/names for TCG and KVM > >>> modes. Otherwise our predefined models get less useful as they will > >>> require low-level feature-bit fiddling on the libvirt side to make them > >>> work as expected.) > >> > >> Agreed. From a user's perspective, I think the more a CPU model "just works", > >> whether it's KVM or TCG, the better. > > > > Yes, that's right. But I think extending the same expectation to "-cpu > > ...,enforce" is not necessary, and perhaps even wrong for "-cpu > > ...,check" since it's only a warning rather than a fatal error. > > > > Paolo > > > > > -- > SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany > GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg -- Eduardo -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list