[libvirt] Modern CPU models cannot be used with libvirt

[Gleb Natapov]

On Fri, Mar 09, 2012 at 03:15:26PM -0600, Anthony Liguori wrote:
> On 03/09/2012 03:04 PM, Daniel P. Berrange wrote:
> >On Fri, Mar 09, 2012 at 05:56:52PM -0300, Eduardo Habkost wrote:
> >>Resurrecting an old thread:
> >>
> >>I didn't see any clear conclusion in this thread (this is why I am
> >>resurrecting it), except that many were arguing that libvirt should
> >>simply copy and/or generate the CPU model definitions from Qemu. I
> >>really don't think it's reasonable to expect that.
> >>
> >>On Thu, Dec 15, 2011 at 03:54:15PM +0100, Jiri Denemark wrote:
> >>>Hi,
> >>>
> >>>Recently I realized that all modern CPU models defined in
> >>>/etc/qemu/target-x86_64.conf are useless when qemu is used through libvirt.
> >>>That's because we start qemu with -nodefconfig which results in qemu ignoring
> >>>that file with CPU model definitions. We have a very good reason for using
> >>>-nodefconfig because we need to control the ABI presented to a guest OS and we
> >>>don't want any configuration file that can contain lots of things including
> >>>device definitions to be read by qemu. However, we would really like the new
> >>>CPU models to be understood by qemu even if used through libvirt. What would
> >>>be the best way to solve this?
> >>>
> >>>I suspect this could have been already discussed in the past but obviously a
> >>>workable solution was either not found or just not implemented.
> >>
> >>So, our problem today is basically:
> >>
> >>A) libvirt uses -nodefconfig;
> >>B) -nodefconfig makes Qemu not load the config file containing the CPU
> >>    model definitions; and
> >>C) libvirt expects the full CPU model list from Qemu to be available.
> >
> >I could have sworn we had this discussion a year ago or so, and had decided
> >that the default CPU models would be in something like /usr/share/qemu/cpu-x86_64.conf
> >and loaded regardless of the -nodefconfig setting. /etc/qemu/target-x86_64.conf
> >would be solely for end user configuration changes, not for QEMU builtin
> >defaults.
> >
> >But looking at the code in QEMU, it doesn't seem we ever implemented this ?
> 
> I don't remember that discussion and really don't think I agree with the conclusion.
> 
> If libvirt wants to define CPU models on their own, they can.  If
It can't without knowing qemu/host cpu/host kernel capabilities and
knowing the logic that qemu uses to combine them.

> libvirt wants to use the user's definitions, don't use -nodefconfig.
> 
> CPU models aren't a QEMU concept.  The reason it's in the
I do not know what do you mean by that, but CPU capabilities (and CPU
model is only a name for a group of them) are KVM/TCG concept and,
by inclusion, are QEMU concept. If QEMU will not have built in support
for CPU models (as a name for a group of CPU capabilities) then how do
you start a guest without specifying full set of CPU capabilities on a
command line?

> configuration file is to allow a user to add their own as they see
> fit.  There is no right model names. It's strictly a policy.
> 
So you think it should be user's responsibility to check what his
qemu/host cpu/host kernel combo can support?

--
			Gleb.