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Re: [Qemu-devel] [Question] why x2apic's set by default without host sup
From: |
Jiri Denemark |
Subject: |
Re: [Qemu-devel] [Question] why x2apic's set by default without host support(on Nehalem CPU). |
Date: |
Tue, 23 Jul 2013 13:27:12 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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