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 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1431473 Title: kvm_intel (nested) module will not load [Input/output error] To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1431473/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs