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Re: [libvirt] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

(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

> >> 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
> > 
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