Reflect in virtiofs.rst that virtiofs can be used without NUMA

Reflect in the virtiofs documentation that virtiofs can now be used
even without NUMA. While at it, be more precise where and why shared
memory is required.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

diff --git a/docs/kbase/virtiofs.rst b/docs/kbase/virtiofs.rst
index dea8e79f833c7cdd2ca7b9012a6487cefca76e70..01440420d76dfae9accaf3442a18748ca51697c9 100644 (file)
--- a/docs/kbase/virtiofs.rst
+++ b/docs/kbase/virtiofs.rst
@@ -16,10 +16,17 @@ 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:
+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, it is possible to specify the memory backend
+without NUMA (using the so called memobject interface).
 
 Either of the following:
 
@@ -46,7 +53,7 @@ Either of the following:
 Guest setup
 ===========
 
-#. Specify the NUMA topology
+#. 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: