Add a document describing the usage of virtiofs.
---
docs/kbase.html.in | 3 +
docs/kbase/virtiofs.rst | 152 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 155 insertions(+)
create mode 100644 docs/kbase/virtiofs.rst
diff --git a/docs/kbase.html.in b/docs/kbase.html.in
index db84b95b60..7055f4fda4 100644
--- a/docs/kbase.html.in
+++ b/docs/kbase.html.in
@@ -33,6 +33,9 @@
<dt><a href="kbase/qemu-passthrough-security.html">Security with QEMU passthrough</a></dt>
<dd>Examination of the security protections used for QEMU and how they need
configuring to allow use of QEMU passthrough with host files/devices.</dd>
+
+ <dt><a href="kbase/virtiofs.html">Virtio-FS</a></dt>
+ <dd>Share a filesystem between the guest and the host</dd>
</dl>
</div>
diff --git a/docs/kbase/virtiofs.rst b/docs/kbase/virtiofs.rst
new file mode 100644
index 0000000000..fe6885d139
--- /dev/null
+++ b/docs/kbase/virtiofs.rst
@@ -0,0 +1,152 @@
+============================
+Sharing files with Virtio-FS
+============================
+
+=== 8< delete before merging 8< ===
+NOTE: if you're looking at this note, this is just a proposal.
+See the up-to-date version on: https://libvirt.org/kbase/virtiofs.html
+=== 8< --------------------- 8< ===
+
+.. contents::
+
+=========
+Virtio-FS
+=========
+
+Virtio-FS 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/
+
+==========
+Host setup
+==========
+
+The host-side virtiofsd daemon, like other vhost-user backed devices,
+requires shared memory between the host and the guest. As of QEMU 4.2, this
+requires specifying a NUMA topology for the guest and explicitly specifying
+a memory backend. Multiple options are available:
+
+Either of the following:
+
+* 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
+
+===========
+Guest setup
+===========
+
+#. Specify the NUMA topology
+
+ 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
+
+ Either of the following:
+
+ * 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>
+
+#. Add the ``vhost-user-fs`` QEMU device via the ``filesystem`` element
+
+ ::
+
+ <domain>
+ ...
+ <devices>
+ ...
+ <filesystem type='mount' accessmode='passthrough'>
+ <driver type='virtiofs'/>
+ <source dir='/path'/>
+ <target dir='mount_tag'/>
+ </filesystem>
+ ...
+ </devices>
+ </domain>
+
+ Note that despite its name, the ``target dir`` is actually a mount tag and does
+ not have to correspond to the desired mount point in the guest.
+
+ So far, ``passthrough`` is the only supported access mode and it requires
+ running the ``virtiofsd`` daemon as root.
+
+#. Boot the guest and mount the filesystem
+
+ ::
+
+ guest# mount -t virtiofs mount_tag /mnt/mount/path
+
+ Note: this requires virtiofs support in the guest kernel (Linux v5.4 or later)
+
+===================
+Optional parameters
+===================
+
+More optional elements can be specified
+
+::
+
+ <driver type='virtiofs' queue='1024'/>
+ <binary path='/usr/libexec/virtiofsd' xattr='on'>
+ <cache mode='always'/>
+ <lock posix_lock='on' flock='on'/>
+ </binary>
--
2.24.1
