Create VM w/ cache=none on tmpfs

Hi, I've seen that in the past, libvirt couldn't start VMs when the disk image was stored on a file system that doesn't support direct I/O having the 'cache=none' configuration [0]. On the KubeVirt project, we have some storage tests on a particular provider which does just that - try to create / start a VM whose disk is on tmpfs and whose definition features 'cache=none'. The behavior we're seeing is that libvirt throws this warning: ``` Unexpected Warning event received: testvmig4zsxc2f8swkxv22xkhx2vrb4ppqbfdfgfgqh5gq8plqzrv5,853ff3d9-70d4-43c5-b9ff-4d5815ea557d: server error. command SyncVMI failed: "LibvirtError(Code=1, Domain=10, Message='internal error: process exited while connecting to monitor: 2020-03-25T10:09:21.656238Z qemu-kvm: -drive file=/var/run/kubevirt-ephemeral-disks/disk-data/disk0/disk.qcow2,format=qcow2,if=none,id=drive-ua-disk0,cache=none: file system may not support O_DIRECT\n2020-03-25T10:09:21.656391Z qemu-kvm: -drive file=/var/run/kubevirt-ephemeral-disks/disk-data/disk0/disk.qcow2,format=qcow2,if=none,id=drive-ua-disk0,cache=none: Could not open backing file: Could not open '/var/run/kubevirt-private/vmi-disks/disk0/disk.img': Invalid argument')" ``` But actually proceeds, and is able to start the VM - but seems it coerces the cache value to writeThrough. Is this the expected behavior ? e.g. cache = none can't be used when the disk images are on a tmpfs file system ? I know it was, not sure about now (libvirt-5.6.0-7) ... [0] - https://bugs.launchpad.net/nova/+bug/959637