[libvirt-users] Why Guest does not retain Host CPU flags

[Jiri Denemark]

On Mon, Feb 06, 2017 at 09:51:06 +0530, akhilesh rawat wrote:
> hi ,
> 
> I am creating Guest by Libvirt tool " virt-install"  with cpu model as
> host.
> 
> After Guest creation i see Guest Missing quite no of flags which Host was
> having .
> 
> What could be the reason for this ?  I am expecting all flags of Host to e
> present in Guest as well when choosing cpu model as host .

Even if you request "host" CPU model, the guest will always see only a
subset of features the host supports. Not all features can be used in a
virtual machine and even if they can be used they usually need some
support form KVM or QEMU or both. That is both KVM and QEMU will filter
out features they cannot currently virtualize and thus you will get a
smaller set in comparison to the host CPU.

Jirka