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QEMU supports a convenient CPU option for enabling all supported hyperv flags (hv-passthrough=on).

libvirt could support that flags, so management software could easily enable all supported hyperv flags out of the box when live migration support isn't required by the user.

Libvirt could also leverage hv-passthrough to figure out which Hyper-V enlightenments
are supported (for https://bugzilla.redhat.com/show_bug.cgi?id=1717611) the same way
it does with host CPU features (running QEMU with "-cpu host" and querying all supported
CPUIDs).

Bulk update: Move RHEL-AV bugs to RHEL9. If necessary to resolve in RHEL8, then clone to the current RHEL8 release.

Proposed patch: https://listman.redhat.com/archives/libvir-list/2021-November/msg00861.html

Merged upstream: https://gitlab.com/libvirt/libvirt/-/commit/8b6766bd77108dc826715a359157d113baaa6f8f

Verify this bug with libvirt-8.0.0-1.el9.x86_64:

1. prepare a guest which have hv-passthrough element:

# virsh dumpxml vm1
...
  <features>
    <acpi/>
    <apic/>
    <hyperv mode='passthrough'>
    </hyperv>
  </features>
...

2. start guest

# virsh start vm1
Domain 'vm1' started


3. check qemu command line and make sure that libvirt add hv-passthrough=on in qemu command line 

# ps aux|grep qemu
... -cpu Skylake-Client-IBRS,...,hv-passthrough=on ...

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 (new packages: libvirt), 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/RHBA-2022:2390