[libvirt] Preferred CPU model not allowed by hypervisor

Jared list-virt at legroom.net
Sat Jul 21 09:46:18 UTC 2012


I'm having a weird problem where libvirt/qemu/kvm won't let me use the model
processor I have defined in my domain's config file.  Instead, I get the
error message in libvirtd.log that:

warning : x86Decode:1346 : Preferred CPU model Nehalem not allowed by
hypervisor; closest supported model will be used

If I review the qemu log for that particular domain, I see that my CPU has
been changed to this:

-cpu kvm64,+lahf_lm,+popcnt,+sse4.2,+sse4.1,+ssse3

(in other places, I see it set to core2duo rather than kvm64)

However, it *should* be Nehalem.  For some background, I'm running lvm on a
westmere proc, which is the successor to Nehalem.  I'm specifying Nehalem as
the target platform, though, to make it easier to migrate to another server
if necessary.  I do have this same problem if I set this to Westmere,
though, so it's not unique to Nehalem.

As for support and capabilities, libvirt correctly detects my host CPU as

$ virsh capabilities | grep model

qemu-kvm does (as far as I can tell) support Nehalem:

$ qemu-kvm -cpu ?model | grep Nehalem
x86          Nehalem  Intel Core i7 9xx (Nehalem Class Core i7)

Nehalem is defined in qmeu's target-x86_64.conf:

grep Nehalem /etc/qemu/target-x86_64.conf
   name = "Nehalem"
   model_id = "Intel Core i7 9xx (Nehalem Class Core i7)"

And if I run a cpu check on the processor, it seems to work fine:

$ qemu-kvm -cpu Nehalem,check
VNC server running on `'

So, I I'm creating the new domain as a virt-install with the qemu-kvm
backend as follows:

virt-install --name=test --ram=1024 --arch=x86_64 --vcpus=2 --cpu=Nehalem
--virt-type=kvm <SNIP>

This results in the following cpu configuration:
  <cpu mode='custom' match='exact'>
    <model fallback='allow'>Nehalem</model>

But then, when I start this domain, I get the error message posted above.

Any ideas what's going on here?  I'm at a loss, and unfortunately I don't
have much experience with kvm/libvirt just yet so I don't know what I should
be focusing on for troubleshooting.  Would appreciate any suggestions or



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