Comment 9 for bug 1431473

I think this is a problem very much tied to the raw host cpu type and the masking of that as core2duo for the guest. I just tried with a current Trusty host with a current Vivid guest masked to core2duo:

host cpu:
processor : 7
vendor_id : GenuineIntel
cpu family : 6
model : 58
model name : Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz
stepping : 9
microcode : 0x19
flags : 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 eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid fsgsbase smep erms

libvirt setting:
  <cpu mode='custom' match='exact'>
    <model fallback='allow'>core2duo</model>
    <vendor>Intel</vendor>
  </cpu>

guest cpu:
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 15
model name : Intel(R) Core(TM)2 Duo CPU T7700 @ 2.40GHz
stepping : 11
microcode : 0x1
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss syscall nx lm constant_tsc rep_good nopl pni vmx ssse3 cx16 x2apic hypervisor lahf_lm vnmi ept

There is some difference of what the guest sees on Scott's host compared to mine:
--- cpuflags-smb 2015-03-17 10:38:33.477207501 +0100
+++ cpuflags-smoser 2015-03-17 10:38:59.741208228 +0100
@@ -23,6 +23,7 @@
 ss
 syscall
 nx
+rdtscp
 lm
 constant_tsc
 rep_good
@@ -31,8 +32,8 @@
 vmx
 ssse3
 cx16
-x2apic
+sse4_1
+sse4_2
+popcnt
 hypervisor
 lahf_lm
-vnmi
-ept