We want to support dynamically using multiple memslots to expose virtio-mem device memory to the VM; using dynamically multiple memslots can drastically reduce memory overhead in the hypervisor (especially, KVM), when a device exposes only comparatively small memory towards the VM, compared to its possible maximum size.

For QEMU, the feature is enabled using "dynamic-memslots=on". With "dynamic-memslots=off" (current default), the feature is disabled and we default to using a single large memslot statically.

In combination with vhost devices, this new feature can be problematic if the devices support less than 509 memslots. If such devices are created before the virtio-mem device in QEMU, virtio-mem will default to the old handling of using a single memslot only. If the devices are created after the virtio-mem devices (on the cmdline, hotplug of such devices), QEMU will bail out.

For vhost support in the kernel, we should always have 509 memslots configured in RHEL using

$ cat /etc/modprobe.d/vhost.conf 
# Increase default vhost memory map limit to match
# KVM's memory slot limit
options vhost max_mem_regions=509

vhost-user based on rust-vmm/vhost supports significantly less memslots (16? 32?) and needs support for more memslots.

Initial support has been merged into QEMU and will be part of QEMU 8.2.0. One bugfix is still pending.