Re: [Qemu-devel] [PATCH 0/2] target-i386: "custom" CPU model + script to dump existing CPU models

[Daniel P. Berrange]

On Tue, Jun 23, 2015 at 07:58:44PM +0200, Andreas Färber wrote:
> Am 23.06.2015 um 19:45 schrieb Eduardo Habkost:
> > On Tue, Jun 23, 2015 at 07:41:50PM +0200, Andreas Färber wrote:
> > [...]
> >> If that is the whole problem here, then why not just add a global flag
> >> to only enable explicitly requested KVM features? All other features
> >> should not depend on the host, and the whole discussion about -x.y seems
> >> like a distraction.
> > 
> > Now replace "KVM features" with "CPU fatures", because all CPU features
> > are KVM features, as all of them depend on KVM code enabling them on
> > GET_SUPPORTED_CPUID.
> > 
> > Thus, the global flag to only enable explicitly request KVM features on
> > CPUs is "-cpu custom", which doesn't enable any CPU feature at all.
> 
> If libvirt wants to use an empty CPU model, then why export our models
> to libvirt?
> 
> I don't mind there being an optional custom model, I mind our
> compat_props getting ignored that way, which are unrelated to adding new
> features, in fact they suppress just that for the -2.3 examples.

If QEMU has a '-cpu custom' there isn't actually any need for QEMU to
export its CPU models to libvirt. Libvirt already has its own CPU
model database, so we'd just start to use our own database exclusively.

Libvirt would have to take care that we don't break ABI when updating
from a QEMU that pre-dates the '-cpu custom' feature - for that we
have done a one-time only extraction of details of all the historical
variations in QEMU CPUS models per machine type.

So apart from backcompat for existing QEMUs in the wild, libvirt would
no longer have any need to know about QEMU's CPU models.

Regards,
Daniel
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