docs/nvdimm: add example on persistent backend setup

Persistent backend setup requires some knowledge about nvdimm and ndctl
tool. Some users report they may struggle to gather these knowledge and
have difficulty to setup it properly.

Here we provide two examples for persistent backend and gives the link
to ndctl. By doing so, user could try it directly and do more
investigation on persistent backend setup with ndctl.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Pankaj Gupta <pagupta@redhat.com>

Message-Id: <20190801004053.7021-1-richardw.yang@linux.intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/nvdimm.txt

@@ -171,6 +171,35 @@ guest software that this vNVDIMM device contains a region that cannot
 accept persistent writes. In result, for example, the guest Linux
 NVDIMM driver, marks such vNVDIMM device as read-only.
 
+Backend File Setup Example
+--------------------------
+
+Here are two examples showing how to setup these persistent backends on
+linux using the tool ndctl [3].
+
+A. DAX device
+
+Use the following command to set up /dev/dax0.0 so that the entirety of
+namespace0.0 can be exposed as an emulated NVDIMM to the guest:
+
+    ndctl create-namespace -f -e namespace0.0 -m devdax
+
+The /dev/dax0.0 could be used directly in "mem-path" option.
+
+B. DAX file
+
+Individual files on a DAX host file system can be exposed as emulated
+NVDIMMS.  First an fsdax block device is created, partitioned, and then
+mounted with the "dax" mount option:
+
+    ndctl create-namespace -f -e namespace0.0 -m fsdax
+    (partition /dev/pmem0 with name pmem0p1)
+    mount -o dax /dev/pmem0p1 /mnt
+    (create or copy a disk image file with qemu-img(1), cp(1), or dd(1)
+     in /mnt)
+
+Then the new file in /mnt could be used in "mem-path" option.
+
 NVDIMM Persistence
 ------------------
 
@@ -212,3 +241,5 @@ References
     https://www.snia.org/sites/default/files/technical_work/final/NVMProgrammingModel_v1.2.pdf
 [2] Persistent Memory Development Kit (PMDK), formerly known as NVML project, home page:
     http://pmem.io/pmdk/
+[3] ndctl-create-namespace - provision or reconfigure a namespace
+    http://pmem.io/ndctl/ndctl-create-namespace.html