Re: [Qemu-devel] [Question] why x2apic's set by default without host support(on Nehalem CPU).

[Jiri Denemark <jdenemar@redhat.com>]

On Tue, Jul 23, 2013 at 10:44:48 +0800, Peter Huang(Peng) wrote:
> > libvirt's "host-passthrough" uses "-cpu host', and it "-cpu host"
> > enables every feature that can be enabled on the host.
> From my test results, I found that even when use host-passthrough mode, VM's
> cpu features are very different from host, this doesn't match what host-passthrough
> mode's explanation.
> 
> libvirt's option exlanation:
> With this mode, the CPU visible to the guest should be exactly  the  same as  the host 
> CPU even in  the aspects  that libvirt  does not understand.

The libvirt documentation is what needs to be updated. While
host-passthrough is asking for a CPU which is as close as possible to
the real host CPU, there are features that need special handling before
they can be provided to a guest. And if the hypervisor does not provide
that handling, it may just filter such feature out. Also if some
features can be efficiently provided to a guest even though the host CPU
does not provide them (x2apic is an example of such feature), they may
be provided to a guest.

Jirka