2 files changed, 106 insertions, 0 deletions
diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index b531cacd35..362e99109e 100644
--- a/docs/nvdimm.txt
+++ b/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
diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst
new file mode 100644
index 0000000000..e77881b26f
--- /dev/null
+++ b/docs/virtio-pmem.rst
@@ -0,0 +1,75 @@
+
+========================
+QEMU virtio pmem
+========================
+
+ This document explains the setup and usage of the virtio pmem device
+ which is available since QEMU v4.1.0.
+
+ The virtio pmem device is a paravirtualized persistent memory device
+ on regular (i.e non-NVDIMM) storage.
+
+Usecase
+--------
+
+  Virtio pmem allows to bypass the guest page cache and directly use
+  host page cache. This reduces guest memory footprint as the host can
+  make efficient memory reclaim decisions under memory pressure.
+
+o How does virtio-pmem compare to the nvdimm emulation supported by QEMU?
+
+  NVDIMM emulation on regular (i.e. non-NVDIMM) host storage does not
+  persist the guest writes as there are no defined semantics in the device
+  specification. The virtio pmem device provides guest write persistence
+  on non-NVDIMM host storage.
+
+virtio pmem usage
+-----------------
+
+  A virtio pmem device backed by a memory-backend-file can be created on
+  the QEMU command line as in the following example:
+
+  -object memory-backend-file,id=mem1,share,mem-path=./virtio_pmem.img,size=4G
+  -device virtio-pmem-pci,memdev=mem1,id=nv1
+
+   where:
+   - "object memory-backend-file,id=mem1,share,mem-path=<image>, size=<image size>"
+     creates a backend file with the specified size.
+
+   - "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem
+     pci device whose storage is provided by above memory backend device.
+
+  Multiple virtio pmem devices can be created if multiple pairs of "-object"
+  and "-device" are provided.
+
+Hotplug
+-------
+
+Virtio pmem devices can be hotplugged via the QEMU monitor. First, the
+memory backing has to be added via 'object_add'; afterwards, the virtio
+pmem device can be added via 'device_add'.
+
+For example, the following commands add another 4GB virtio pmem device to
+the guest:
+
+ (qemu) object_add memory-backend-file,id=mem2,share=on,mem-path=virtio_pmem2.img,size=4G
+ (qemu) device_add virtio-pmem-pci,id=virtio_pmem2,memdev=mem2
+
+Guest Data Persistence
+----------------------
+
+ Guest data persistence on non-NVDIMM requires guest userspace applications
+ to perform fsync/msync. This is different from a real nvdimm backend where
+ no additional fsync/msync is required. This is to persist guest writes in
+ host backing file which otherwise remains in host page cache and there is
+ risk of losing the data in case of power failure.
+
+ With virtio pmem device, MAP_SYNC mmap flag is not supported. This provides
+ a hint to application to perform fsync for write persistence.
+
+Limitations
+------------
+- Real nvdimm device backend is not supported.
+- virtio pmem hotunplug is not supported.
+- ACPI NVDIMM features like regions/namespaces are not supported.
+- ndctl command is not supported.