VirtIO Pmem

crosvm supports virtio-pmem to provide a virtual device emulating a byte-addressable persistent memory device. The disk image is provided to the guest using a memory-mapped view of the image file, and this mapping can be directly mapped into the guest's address space if the guest operating system and filesystem support DAX.

Pmem devices may be added to crosvm using the --pmem flag, specifying the filename of the backing image as the parameter. By default, the pmem device will be writable; add ro=true to create a read-only pmem device instead.

crosvm run \
  --pmem disk.img \
  ... # usual crosvm args

The Linux virtio-pmem driver can be enabled with the CONFIG_VIRTIO_PMEM option. It will expose pmem devices as /dev/pmem0, /dev/pmem1, etc., which may be mounted like any other block device. A pmem device may also be used as the root filesystem by adding root=true to the --pmem flag:

crosvm run \
  --pmem rootfs.img,root=true,ro=true \
  ... # usual crosvm args

The advantage of pmem over a regular block device is the potential for less cache duplication; since the guest can directly map pages of the pmem device, it does not need to perform an extra copy into the guest page cache. This can result in lower memory overhead versus virtio-block (when not using O_DIRECT).

The file backing a persistent memory device is mapped directly into the guest's address space, which means that only the raw disk image format is supported; disk images in qcow2 or other formats may not be used as a pmem device. See the block device for an alternative that supports more file formats.