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hi there,

In my test environment, i created a vm and configured the cpu with 
host-model, when I migrate the vm to another host with the same cpu, it 
failed the migration compatibility check which complains the cpu 
definition of domain is incompatible with target host cpu.

As we know, when the domain configured as above starts, the host-model 
cpu definition will automatically converted to custom cpu model and with 
some addtional features that the KVM supported, these addtional features 
may contains features that the host doesn't support.

In the code, the compatibility of the target host is check by calling 
compareCPU()(libvirt API). The compareCPU() can only recongnize the 
features probed by cpuid instruction on the host, but it may not 
recognize the features of cpu definition of domain xml (virsh dumpxml 
domainname) when the domain running. So the compatibility check will 
fail when KVM support one or more features which is considerd as 
disabled by the cpuid instuction.

I think we should call compareHypervisorCPU() or something like that 
(supported by libvirt since v4.4.0) instead of compareCPU() to check the 
migration compatibility.


My test environment is as follow:
host cpu: Cascadelake
libvirt-6.9
qemu-5.0

host-model cpu:
    <mode name='host-model' supported='yes'>
       <model fallback='forbid'>Cascadelake-Server</model>
       <vendor>Intel</vendor>
       <feature policy='require' name='ss'/>
       <feature policy='require' name='hypervisor'/>
       <feature policy='require' name='tsc_adjust'/>
       <feature policy='require' name='umip'/>
       <feature policy='require' name='pku'/>
       <feature policy='require' name='md-clear'/>
       <feature policy='require' name='stibp'/>
       <feature policy='require' name='arch-capabilities'/>
       <feature policy='require' name='xsaves'/>
       <feature policy='require' name='invtsc'/>
       <feature policy='require' name='rdctl-no'/>
       <feature policy='require' name='ibrs-all'/>
       <feature policy='require' name='skip-l1dfl-vmentry'/>
       <feature policy='require' name='mds-no'/>
       <feature policy='require' name='pschange-mc-no'/>
       <feature policy='disable' name='hle'/>
       <feature policy='disable' name='rtm'/>
     </mode>


The hypervisor, umip, pschange-mc-no features block the compatibility check


-- 
Best Regards,
Guoyi Tu