+++ This bug was initially created as a clone of Bug #1761678 +++

Description of problem:
Wrongly support "Cascadelake-Server" on physical host without avx512_vnni cpu flag

Version-Release number of selected component (if applicable):
libvirt-4.5.0-35.module+el8.1.0+4227+b2722cb3.x86_64
qemu-kvm-2.12.0-88.module+el8.1.0+4233+bc44be3f.x86_64
kernel-4.18.0-147.el8.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Check physical host cpu info
# lscpu
...
Model name:          Intel(R) Xeon(R) Gold 6130 CPU @ 2.10GHz
...
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 pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 invpcid_single pti intel_ppin ssbd mba ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm mpx rdt_a avx512f avx512dq rdseed adx smap clflushopt clwb intel_pt avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts pku ospke md_clear flush_l1d

# lscpu |grep avx512_vnni
No output

2. Check "virsh capabilities" and "virsh domcapabilities"
# virsh capabilities
<capabilities>

  <host>
    <uuid>4c4c4544-0044-3210-8034-cac04f4e5232</uuid>
    <cpu>
      <arch>x86_64</arch>
      <model>Skylake-Server-IBRS</model>
      <vendor>Intel</vendor>
      <microcode version='33554526'/>
      <topology sockets='1' cores='16' threads='2'/>
...

# virsh domcapabilities
...
  <cpu>
    <mode name='host-passthrough' supported='yes'/>
    <mode name='host-model' supported='yes'>
      <model fallback='forbid'>Cascadelake-Server</model>
      <vendor>Intel</vendor>
      <feature policy='require' name='ss'/>
      <feature policy='require' name='vmx'/>
      <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='skip-l1dfl-vmentry'/>
      <feature policy='disable' name='avx512vnni'/>
    </mode>
    <mode name='custom' supported='yes'>
...
      <model usable='yes'>Skylake-Server</model>
      <model usable='yes'>Skylake-Server-IBRS</model>
      <model usable='yes'>Skylake-Client</model>
      <model usable='yes'>Skylake-Client-IBRS</model>
...
      <model usable='no'>Cascadelake-Server</model>
...
    </mode>
  </cpu>
...

3. Start a shutdown VM with the following conf
# virsh domstate avocado-vt-vm1 
shut off

# virsh dumpxml avocado-vt-vm1 |grep "<cpu" -A2
  <cpu mode='host-model' check='partial'>
    <model fallback='allow'/>
  </cpu>

# virsh start avocado-vt-vm1
Domain avocado-vt-vm1 started

# virsh dumpxml avocado-vt-vm1 |grep "<cpu" -A20
  <cpu mode='custom' match='exact' check='full'>
    <model fallback='forbid'>Cascadelake-Server</model>
    <vendor>Intel</vendor>
    <feature policy='require' name='ss'/>
    <feature policy='require' name='vmx'/>
    <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='skip-l1dfl-vmentry'/>
    <feature policy='disable' name='avx512vnni'/>
    <feature policy='disable' name='mpx'/>
  </cpu>

# ps -ef |grep avocado-vt-vm1
-cpu Cascadelake-Server,ss=on,vmx=on,hypervisor=on,tsc-adjust=on,umip=on,pku=on,md-clear=on,stibp=on,arch-capabilities=on,xsaves=on,skip-l1dfl-vmentry=on,avx512vnni=off

# virsh console avocado-vt-vm1
onnected to domain avocado-vt-vm1
Escape character is ^]

Red Hat Enterprise Linux 8.1 (Ootpa)
Kernel 4.18.0-147.el8.x86_64 on an x86_64

localhost login: root
Password: 
Last login: Tue Oct 15 02:45:16 from 192.168.122.1
[root@localhost ~]# lscpu 
...
Model name:          Intel Xeon Processor (Cascadelake)
...
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 pdpe1gb rdtscp lm constant_tsc rep_good nopl xtopology cpuid tsc_known_freq pni pclmulqdq vmx ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch cpuid_fault invpcid_single pti ssbd ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm avx512f avx512dq rdseed adx smap clflushopt clwb avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves arat umip pku ospke md_clear arch_capabilities

[root@localhost ~]# lscpu |grep avx512_vnni
No output

Actual results:
As step-2 and step-3 show.

Expected results:
Since the physical host does not support Cascadelake-Server cpu model and the "virsh domcapabilities" also shows this model is not supported. however, the default for host-mode cpu conf is Cascadelake-Server and VM can start successfully with this kind of conf although without avx512_vnni enabled.

Additional info:

--- Additional comment from Jiri Denemark on 2019-10-16 07:27:37 UTC ---

The strange behavior is caused by insufficient CPU signature checks in
libvirt. We only consider family and model parts of the signuture, which is
the same for both Skylake-Server and Cascadelake-Server CPUs (family 6, model
85). The two CPUs differ only in stepping which is ignored by libvirt.

--- Additional comment from Jiri Denemark on 2020-04-14 12:23:05 UTC ---

This was fixed in a series of commits which ends with

commit 5d6059f8ec16d64f240dc5e6413ca55a3b46b3f7
Refs: v6.2.0-111-g5d6059f8ec
Author:     Jiri Denemark <jdenemar>
AuthorDate: Thu Mar 26 21:55:14 2020 +0100
Commit:     Jiri Denemark <jdenemar>
CommitDate: Wed Apr 8 17:52:50 2020 +0200

    cpu_map: Distinguish Cascadelake-Server from Skylake-Server

    The signatures of these two CPU model differ only in stepping as both
    report family 6 and model 85. Skylake-Server uses stepping 4 or less and
    Cascadelake-Server uses stepping 5..7.

    https://bugzilla.redhat.com/show_bug.cgi?id=1761678

    Signed-off-by: Jiri Denemark <jdenemar>
    Reviewed-by: Ján Tomko <jtomko>