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

[Peter Huang(Peng)]

Hi, Marcelo

Thanks for replying.

I noticed that, kvm has emulated x2apic, but I can't find where the code exits.
Could you give me some hint?
Thanks again.


On 2013-07-23 7:55, Marcelo Tosatti wrote:
> Peter,
>
> Because x2apic support is emulated in software.
>
> On Mon, Jul 22, 2013 at 08:36:05PM +0800, Peter Huang(Peng) wrote:
>> I traced the qemu process, and find out that host cpuid doesn't support 
>> x2apic,
>> but VM interception seems enabled x2apic.
>>
>> with function=1(bit 21 indicates x2apic)
>> host_cpuid:ecx=0x009ce3bd
>> guest_cpuid:ecx=0x00b82201
>>
>> On 2013-07-22 19:24, Peter Huang(Peng) wrote:
>>> Hi, Everyone
>>>
>>> I have been encountered a problem recently.
>>> My box uses Nehalem E5520 CPU, and this model doesn't support x2apic 
>>> feature.
>>> I created a VM on KVM, and uses cpu mode=pass-through, I can get the right 
>>> cpu
>>> model from the VM, but I can also see that the CPU in the vm supports 
>>> x2apic.
>>>
>>> This seems werid.
>>> From QEMU's CPU model definition, Nehalem also doesn't support x2apic.
>>>
>>> Can anyone explain it?
>>>
>>> Thanks to you all!
>>>
>>> Below is the cpuinfo from host and guest:
>>> host:
>>> fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 
>>> clflush
>>>  dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx rdtscp lm constant_tsc
>>> arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf pni 
>>> dtes64
>>> monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm dca sse4_1 sse4_2
>>> popcnt lahf_lm dtherm tpr_shadow vnmi flexpriority ept vpid
>>>
>>> vm:
>>> fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 
>>> clflush
>>> mmx fxsr sse sse2 ss ht syscall nx rdtscp lm constant_tsc arch_perfmon 
>>> rep_good
>>>  nopl pni ssse3 cx16 sse4_1 sse4_2 *x2apic *popcnt hypervisor lahf_lm