I had this issue (kernel oops) when I explicitly disabled "ignore_msrs"
(Ubuntu Trusty, 3.13.0-54-generic):

cat /etc/modprobe.d/qemu-system-x86.conf 
options kvm-intel nested=y ept=y
options kvm ignore_msrs=1

Removing the option "kvm ignore_msrs=1" made the guest (L2) run but give
a "KVM: entry failed, hardware error 0x7" when the L1 guest (guest
hypervisor) was booted with these cpu flags (libvirt):

<cpu> <arch>x86_64</arch> <model>Nehalem</model> <vendor>Intel</vendor>
<topology sockets='1' cores='2' threads='2'/> <feature name='rdtscp'/>
<feature name='dca'/> <feature name='pdcm'/> <feature name='xtpr'/>
<feature name='tm2'/> <feature name='est'/> <feature name='vmx'/>
<feature name='ds_cpl'/> <feature name='monitor'/> <feature
name='dtes64'/> <feature name='pbe'/> <feature name='tm'/> <feature
name='ht'/> <feature name='ss'/> <feature name='acpi'/> <feature
name='ds'/> <feature name='vme'/> </cpu>

Guest VM (L2) running fine with these cpu flags (libvirt) for guest VM
(L1):

<cpu match='exact'> <cpu mode='host-passthrough'/>
<model>Nehalem</model> <feature policy='require' name='vmx'/> </cpu>

@Serge Hallyn: I wonder what cpu parameters you have defined for your L1
guest (guest hypervisor)

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https://bugs.launchpad.net/bugs/1448269

Title:
  qemu guest hangs on nested kvm startup with host kernel oops

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