[libvirt] Error when creating VM with persistent memory

I am trying to create a VM with added persistent memory using virsh. It fails when persistent memory namespace size is larger than available system memory. Please see error below, prealloc=yes is set in the command line. For dax type namespace, as I understand, prealloc should not be needed. Is this support to add persistent memory implemented in virsh? If so, how to set prealloc=no? Anything else needed to add in xml for persistent memory? Thanks. 1. dax namespace (400GB) which is larger than system memory. # ndctl create-namespace -t pmem -m devdax --align=4k -s 400G { "dev":"namespace1.0", "mode":"devdax", "map":"dev", "size":"393.75 GiB (422.78 GB)", "uuid":"00670b70-7871-4749-a901-9184b7a388d8", "raw_uuid":"a5a1d3f8-01e4-45a2-bad1-38fd09bf4672", "daxregion":{ "id":1, "size":"393.75 GiB (422.78 GB)", "align":4096, "devices":[ { "chardev":"dax1.0", "size":"393.75 GiB (422.78 GB)" } ] }, "numa_node":1 } 2. Relevant parts of xml <domain type='kvm'> <name>test</name> <uuid>02c49a19-ce9e-4320-9a6b-1ebf0913a10e</uuid> <maxMemory slots='16' unit='KiB'>459800576</maxMemory> <memory unit='KiB'>65536000</memory> <currentMemory unit='KiB'>65536000</currentMemory> <vcpu placement='static'>4</vcpu> <os> : : <cpu mode='custom' match='exact' check='partial'> <feature policy='require' name='hypervisor'/> <numa> <cell id='0' cpus='0-3' memory='65536000' unit='KiB'/> </numa> </cpu> : : <memory model='nvdimm'> <source> <path>/dev/dax1.0</path> </source> <target> <size unit='GiB'>376</size> <node>0</node> <label> <size unit='KiB'>128</size> </label> </target> <address type='dimm' slot='0'/> </memory> 3. Error: # virsh start test error: Failed to start domain test error: internal error: qemu unexpectedly closed the monitor: ftruncate: Invalid argument 2019-08-22T04:16:08.744402Z qemu-system-x86_64: -object memory-backend-file,id=memnvdimm0,prealloc=yes,mem-path=/dev/dax1.0,size=403726925824: unable to map backing store for guest RAM: Cannot allocate memory Virsh version 4.7.0, qemu version 3.0.0