Sharing files with Virtiofs
Virtiofs
Virtiofs is a shared file system that lets virtual machines access a directory tree on the host. Unlike existing approaches, it is designed to offer local file system semantics and performance.
See https://virtio-fs.gitlab.io/
Note: virtiofs currently does not support migration so operations such as migration, save/managed-save, or snapshots with memory are not supported if a VM has a virtiofs filesystem connected.
Optional parameters
More optional elements can be specified
<filesystem type='mount' accessmode='passthrough'> <driver type='virtiofs' queue='1024'/> ... <binary path='/usr/libexec/virtiofsd' xattr='on'> <cache mode='always'/> <lock posix='on' flock='on'/> </binary> </filesystem>
Externally-launched virtiofsd
Libvirtd can also connect the vhost-user-fs device to a virtiofsd daemon launched outside of libvirtd. In that case socket permissions, the mount tag and all the virtiofsd options are out of libvirtd's control and need to be set by the application running virtiofsd.
<filesystem type='mount'> <driver type='virtiofs' queue='1024'/> <source socket='/var/virtiofsd.sock'/> <target dir='tag'/> </filesystem>
Other options for vhost-user memory setup
The following information is necessary if you are using older versions of QEMU and libvirt or have special memory backend requirements.
Almost all virtio devices (all that use virtqueues) require access to at least certain portions of guest RAM (possibly policed by DMA). In case of virtiofsd, much like in case of other vhost-user (see https://www.qemu.org/docs/master/interop/vhost-user.html) virtio devices that are realized by an userspace process, this in practice means that QEMU needs to allocate the backing memory for all the guest RAM as shared memory. As of QEMU 4.2, it is possible to explicitly specify a memory backend when specifying the NUMA topology. This method is however only viable for machine types that do support NUMA. As of QEMU 5.0.0 and libvirt 6.9.0, it is possible to specify the memory backend without NUMA (using the so called memobject interface).
Set up the memory backend
Use memfd memory
No host setup is required when using the Linux memfd memory backend.
Use file-backed memory
Configure the directory where the files backing the memory will be stored with the memory_backing_dir option in /etc/libvirt/qemu.conf
# This directory is used for memoryBacking source if configured as file. # NOTE: big files will be stored here memory_backing_dir = "/dev/shm/"
Use hugepage-backed memory
Make sure there are enough huge pages allocated for the requested guest memory. For example, for one guest with 2 GiB of RAM backed by 2 MiB hugepages:
# virsh allocpages 2M 1024
Specify the NUMA topology (this step is only required for the NUMA case)
in the domain XML of the guest. For the simplest one-node topology for a guest with 2GiB of RAM and 8 vCPUs:
<domain> ... <cpu ...> <numa> <cell id='0' cpus='0-7' memory='2' unit='GiB' memAccess='shared'/> </numa> </cpu> ... </domain>
Note that the CPU element might already be specified and only one is allowed.
Specify the memory backend
One of the following:
memfd memory
<domain> ... <memoryBacking> <source type='memfd'/> <access mode='shared'/> </memoryBacking> ... </domain>
File-backed memory
<domain> ... <memoryBacking> <access mode='shared'/> </memoryBacking> ... </domain>
This will create a file in the directory specified in qemu.conf
Hugepage-backed memory
<domain> ... <memoryBacking> <hugepages> <page size='2' unit='M'/> </hugepages> <access mode='shared'/> </memoryBacking> ... </domain>