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

[Marcelo Tosatti <mtosatti@redhat.com>]

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