system/devices/virtio-pmem.rst

3 virtio pmem
6 This document explains the setup and usage of the virtio pmem device.
7 The virtio pmem device is a paravirtualized persistent memory device
8 on regular (i.e non-NVDIMM) storage.
11 -------
13 Virtio pmem allows to bypass the guest page cache and directly use
17 How does virtio-pmem compare to the nvdimm emulation?
18 -----------------------------------------------------
20 NVDIMM emulation on regular (i.e. non-NVDIMM) host storage does not
21 persist the guest writes as there are no defined semantics in the device
22 specification. The virtio pmem device provides guest write persistence
23 on non-NVDIMM host storage.
25 virtio pmem usage
26 -----------------
28 A virtio pmem device backed by a memory-backend-file can be created on
31     -object memory-backend-file,id=mem1,share,mem-path=./virtio_pmem.img,size=4G
32     -device virtio-pmem-pci,memdev=mem1,id=nv1
36   - "object memory-backend-file,id=mem1,share,mem-path=<image>, size=<image size>"
39   - "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem
40     pci device whose storage is provided by above memory backend device.
42 Multiple virtio pmem devices can be created if multiple pairs of "-object"
43 and "-device" are provided.
46 -------
48 Virtio pmem devices can be hotplugged via the QEMU monitor. First, the
49 memory backing has to be added via 'object_add'; afterwards, the virtio
50 pmem device can be added via 'device_add'.
52 For example, the following commands add another 4GB virtio pmem device to
55  (qemu) object_add memory-backend-file,id=mem2,share=on,mem-path=virtio_pmem2.img,size=4G
56  (qemu) device_add virtio-pmem-pci,id=virtio_pmem2,memdev=mem2
59 ----------------------
61 Guest data persistence on non-NVDIMM requires guest userspace applications
67 With virtio pmem device, MAP_SYNC mmap flag is not supported. This provides
71 -----------
73 - Real nvdimm device backend is not supported.
74 - virtio pmem hotunplug is not supported.
75 - ACPI NVDIMM features like regions/namespaces are not supported.
76 - ndctl command is not supported.