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

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

[Peter Huang(Peng)]

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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

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

[Marcelo Tosatti]

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

Re: [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