virtio-mem will only be supported with selected vhost-user devices. In particular, some devices (including DPDK, SPDK) might need adjustments to even be compatible with memslots that correspond to sparse memory regions.

virtiofsd, which uses the rust-vmm/vhost, now supports 509 memslots is compatible with virtio-mem, including its "dynamic-memslots=on" feature. virtio-mem+virtiofsd will be supported in RHEL 9.4.

Support for qemu-storage-daemon, which uses libvhost-user, is on its way. virtio-mem+qemu-storage-daemon will be supported in RHEL 9.4 or 9.5, depending on when the libvhost-user changes land.

As we will only support selected vhost-user devices with virtio-mem that are compatible with "dynamic-memslots=on", let's default-enable "dynamic-memslots=on" in QEMU RHEL 9.4.

If there is the need to turn the feature off, for example, to make use of some unsupported vhost-user devices, that can be done on the libvirt level..

Note that the feature itself was tested as part of RHEL-15315.