virtio pmem: user document

This patch documents the steps to use virtio pmem.
It also documents other useful information about
virtio pmem e.g use-case, comparison with Qemu NVDIMM
backend and current limitations.

Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Message-Id: <20190821121624.5382-1-pagupta@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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.