Oh yeah, we backported (from upstream libvirt 5.5.0) support for MSR features
to libvirt-4.5.0-29.el8 (i.e., for RHEL-8.1.0), which also included patches to
check CPU features with QEMU by name rather than by their CPUID bits.
Unfortunately, this broke support for hyperv and kvm specific features and
several commits fixing them were pushed later to 5.6.0. However, these
additional patches were not backported to RHEL (they are only included in
RHEL-AV). So once qemu-kvm backported their part of the support (specifically
the unavailable-features QOM property) to qemu-kvm-2.12.0-86, libvirt started
to be broken in the same way it was broken with QEMU 4.1.0 upstream.

The following patches are candidates for backporting to RHEL-8:

    0ccdd476bb qemu: Fix hyperv features with QEMU 4.1
    d99e8f01c7 qemu: Prefer dashes for hyperv features
    1ddf014fef cpu: Drop KVM_ from hyperv feature macros
    9e6172937f cpu: Drop unused KVM features
    4c62ed6068 qemu: Fix KVM features with QEMU 4.1

This is really annoying, I hope we can get a fixed package soon.

The only workaround that I can find is to downgrade to 8.0 version of libvirt. If using centos this can be done like:

dnf install $(rpm -qa |grep libvirt| sed 's/-35.*/-24.3.module_el8.0.0+189+f9babebb.x86_64.rpm/' | sed s',^,http://mirror.centos.org/centos/8.0.1905/AppStream/x86_64/os/Packages/,')

Reproduce this issue with libvirt-4.5.0-38.module+el8.2.0+5435+dd02dc4c.x86_64.

Version:
qemu-kvm-2.12.0-98.module+el8.2.0+5698+10a84757.x86_64
kernel-4.18.0-178.el8.x86_64
libvirt-4.5.0-38.module+el8.2.0+5435+dd02dc4c.x86_64

Steps:
1. Prepare a shutdown VM
# virsh domstate test82 
shut off

2. Start VM with runtime hyperv feature
[root@dell-per730-36 ~]# virsh dumpxml test82 |grep "<hyperv" -A2
    <hyperv>
      <runtime state='on'/>
    </hyperv>

# virsh start test82 
error: Failed to start domain test82
error: unsupported configuration: host doesn't support hyperv 'runtime' feature

3. Start VM with synic hyperv feature
# virsh dumpxml test82 |grep "<hyperv" -A2
    <hyperv>
      <synic state='on'/>
    </hyperv>

# virsh start test82 
error: Failed to start domain test82
error: unsupported configuration: host doesn't support hyperv 'synic' feature

4. Start VM with stimer hyperv feature
# virsh dumpxml test82 |grep "<hyperv" -A2
    <hyperv>
      <stimer state='on'/>
    </hyperv>

# virsh start test82 
error: Failed to start domain test82
error: unsupported configuration: host doesn't support hyperv 'stimer' feature

5. Start VM with frequencies hyperv feature
# virsh dumpxml test82 |grep "<hyperv" -A2
    <hyperv>
      <frequencies state='on'/>
    </hyperv>

# virsh start test82 
error: Failed to start domain test82
error: unsupported configuration: host doesn't support hyperv 'frequencies' feature

Verified this issue with libvirt-4.5.0-39.module+el8.2.0+5690+f1eb5920.x86_64.
1. Update libvirt and restart libvirtd
# yum update libvirt* -y

# systemctl restart libvirtd

# rpm -qa libvirt
libvirt-4.5.0-39.module+el8.2.0+5690+f1eb5920.x86_64

2. Start a VM with runtime/synic/stimer/frequencies hyperv features
# virsh domstate test82 
shut off

# virsh dumpxml test82 |grep "<hyperv" -A5
    <hyperv>
      <runtime state='on'/>
      <synic state='on'/>
      <stimer state='on'/>
      <frequencies state='on'/>
    </hyperv>

# virsh start test82 
Domain test82 started

# ps -ef |grep test82
...
-cpu Haswell-noTSX-IBRS,vme=on,ss=on,vmx=on,f16c=on,rdrand=on,hypervisor=on,arat=on,tsc-adjust=on,umip=on,md-clear=on,stibp=on,arch-capabilities=on,ssbd=on,xsaveopt=on,pdpe1gb=on,abm=on,ibpb=on,skip-l1dfl-vmentry=on,** hv-runtime,hv-synic,hv-stimer,hv-frequencies **

3. Change the "state" parameter of these hyperv features
# virsh dumpxml test82 |grep "<hyperv" -A5
    <hyperv>
      <runtime state='off'/>
      <synic state='off'/>
      <stimer state='off'/>
      <frequencies state='off'/>
    </hyperv>

# virsh destroy test82 ;virsh start test82
Domain test82 destroyed

Domain test82 started

# ps -ef |grep test
...
-cpu Haswell-noTSX-IBRS,vme=on,ss=on,vmx=on,f16c=on,rdrand=on,hypervisor=on,arat=on,tsc-adjust=on,umip=on,md-clear=on,stibp=on,arch-capabilities=on,ssbd=on,xsaveopt=on,pdpe1gb=on,abm=on,ibpb=on,skip-l1dfl-vmentry=on -m 1024  [No hy-* here now.]

All the test result are as expected, move this bug to be verified.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHEA-2020:1587