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

[Peter Huang(Peng)]

Hi,Jiri

Thanks for replying.

So why VM identified different features from host is due to the hypervisor 
filtering.
I will dig into the hypervisor of how it filter features, and update status 
later.

Thanks again.


On 2013-07-23 19:27, Jiri Denemark wrote:
> 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
>