The domain cannot be started when specifying different processor

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The domain cannot be started when specifying different processor

Symptom

Running on Intel Nehalem (or older Penryn) processor, a KVM domain is created using virt-manager. After instalation, the domain's processor is changed to match the host's CPU. The domain is then unable to start:

2012-02-06 17:49:15.985+0000: 20757: error : qemuBuildCpuArgStr:3565 : internal error guest CPU is not compatible with host CPU

Moreover, clicking "Copy host cpu configuration" in virt-manager will show "Pentium III" instead of Nehalem/Penryn.

Virt manager window

Investigation

In /usr/share/libvirt/cpu_map.xml we can see which flags define what CPU model. If we take a look at Nehalem/Penryn definition, we can see this:

<feature name='nx'/>

which means nx flag needs to be presented to identify CPU as Nehalem/Penryn. However, taking look into /proc/cpuinfo we can see this flag is missing.

Solution

Nearly all new BIOSes allow to enable or disable 'No eXecute' bit. However, if disabled some CPUs doesn't report this flag and thus libvirt detects different CPU. Enabling this functionality will then make libvirt report the correct CPU.