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+
+===========
+virtio pmem
+===========
+
+This document explains the setup and usage of the virtio pmem device.
+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.
+
+How does virtio-pmem compare to the nvdimm emulation?
+-----------------------------------------------------
+
+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.