(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@redhat.com> 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