What were you trying to do that didn't work?

With libvirt-11.9.0 validation of hyperv features was introduced to libvirt. But in case of some very old machine types (e.g. pc-i440fx-rhel7.6.0) these checks do not reflect QEMU's behavior. Namely: synic does not require vpindex nor does stimer. It's possible to start QEMU with synic and stimer and without vpindex:

/usr/libexec/qemu-kvm -name guest=QEMUGuest1,debug-threads=on -S -machine pc-i440fx-rhel7.6.0,usb=off,dump-guest-core=off,acpi=on -accel kvm -cpu qemu64,hv-time=on,hv-relaxed=on,hv-vapic=on,hv-spinlocks=0x1fff,hv-synic=on,hv-stimer=on,hv-crash
qemu-kvm: warning: Machine type 'pc-i440fx-rhel7.6.0' is deprecated: machines from the previous RHEL major release are subject to deletion in the next RHEL major release
qemu-kvm: warning: CPU model qemu64-x86_64-cpu is deprecated – use at least 'Nehalem' / 'Opteron_G4', or 'host' / 'max'
VNC server running on ::1:5900

This breaks migration from older libvirt to newer.

What is the impact of this issue to you?

Please provide the package NVR for which the bug is seen:

libvirt-daemon-driver-qemu-11.10.0-2.el9

How reproducible is this bug?:

always

Steps to reproduce

1. Create a domain with the following set of hyperv features:

<hyperv>
  <relaxed state='on'/>
  <vapic state='on'/>
  <spinlocks state='on' retries='8191'/>
  <synic state='on'/>
  <stimer state='on'/>
</hyperv>

1. Migrate it to a host with affected libvirt

Expected results

Migration succeeds

Actual results

Migration fails

BTW: this is a downstream 'clone' of an upstream issue reported here:

https://gitlab.com/libvirt/libvirt/-/issues/837