[libvirt] Question about "f16c", "rdrand" and "vme" in cpu_map.xml

Hi all, "f16c", "rdrand" and "vme" are all standard features, the IvyBridge CPU model has these three features in the file of src/cpu/cpu_map.xml, but the higher model of Haswell-noTSX, Haswell, Broadwell-noTSX and Broadwell do not, which makes it incompatible. BTW, I noticed that "f16c", "rdrand" and "vme" are a part of features of Haswell-noTSX, Haswell, Broadwell-noTSX and Broadwell in QEMU. The commit 732d3d147adecb867f62c67ab3743c0aefd87e50(which introduces the Haswell-noTSX CPU model in libvirt) said that: "Inheritance among CPU model is cool but it makes reviewing CPU model definitions and comparing them to CPU models from QEMU rather hard and unpleasant." I do not get the point "it makes reviewing CPU model definitions and comparing them to CPU models from QEMU rather hard and unpleasant" clearly. Could anyone explain it more explicitly? Looking forward to your replay. Thanks, Jay