This feature comes from Gavin shan’s contribution 
https://issues.redhat.com/browse/RHEL-19738
This feature is enabled at the qemu layer (qemu-kvm-8.2.0-3.el9)

There are 3 high memory regions for GICv3 or GICv4 redistributor, PCI
ECAM and PCI MMIO. Each of them has a property introduced by upstream
commit 6a48c64e ("hw/arm/virt: Add properties to disable high memory
regions") so that the corresponding high memory region can be disabled.

It's notable that another property ("compact-highmem") introduced by
upstream commit f40408a9

("hw/arm/virt: Add 'compact-highmem' property")
so that the compact high memory region layout during assignment can be
disabled, compatible to the old machine types. However, we don't have
the compatible issue since the compact high memory region layout is
always kept as disabled until RHEL9.2.0 machine type and onwards.

Expose those 3 properties: "highmem-redists", "highmem-ecam" and
"highmem-mmio". The property "compact-highmem" is kept as hidden.

Need to expose those properties from libvirt side either.