My reply by email has not arrived by now, hence I'll post it via the archive site. Sorry for the potential double post. Older bootloaders were not split into separate _CODE.fd and _VARS.fd, i.e. there was no separate nvram for the latter file to create. The guest could write to the single bootloader, which then must not be shared. You mark such bootloaders readonly=no and make a copy of the pflash, e.g. next to the VM's disk files, as you did in your TEST ONLY run. It is a mode of operation supported by the formatdomain documentation on the loader element and intended to work as described. This patch makes such combination of parameters actually succeed on Ubuntu, which I find should be useful to the project. In the VMs we use this for, they do not actually write anything to the loader. This meant that we never noticed the bug, which was present in focal and configured qemu to open the loader read-only anyway. But it failed on AA in noble since the bug is now fixed in newer libvirt. As a workaround, we've in the mean time switched to marking the loader stateless and read-only, which is now allowed in libvirt, and also works without requiring a separate nvram. Of course, this only works because the VM does not make any writes and would fail in case it needed to.
