1
2========================
3QEMU virtio pmem
4========================
5
6 This document explains the setup and usage of the virtio pmem device
7 which is available since QEMU v4.1.0.
8
9 The virtio pmem device is a paravirtualized persistent memory device
10 on regular (i.e non-NVDIMM) storage.
11
12Usecase
13--------
14
15  Virtio pmem allows to bypass the guest page cache and directly use
16  host page cache. This reduces guest memory footprint as the host can
17  make efficient memory reclaim decisions under memory pressure.
18
19o How does virtio-pmem compare to the nvdimm emulation supported by QEMU?
20
21  NVDIMM emulation on regular (i.e. non-NVDIMM) host storage does not
22  persist the guest writes as there are no defined semantics in the device
23  specification. The virtio pmem device provides guest write persistence
24  on non-NVDIMM host storage.
25
26virtio pmem usage
27-----------------
28
29  A virtio pmem device backed by a memory-backend-file can be created on
30  the QEMU command line as in the following example:
31
32  -object memory-backend-file,id=mem1,share,mem-path=./virtio_pmem.img,size=4G
33  -device virtio-pmem-pci,memdev=mem1,id=nv1
34
35   where:
36   - "object memory-backend-file,id=mem1,share,mem-path=<image>, size=<image size>"
37     creates a backend file with the specified size.
38
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.
41
42  Multiple virtio pmem devices can be created if multiple pairs of "-object"
43  and "-device" are provided.
44
45Hotplug
46-------
47
48Virtio pmem devices can be hotplugged via the QEMU monitor. First, the
49memory backing has to be added via 'object_add'; afterwards, the virtio
50pmem device can be added via 'device_add'.
51
52For example, the following commands add another 4GB virtio pmem device to
53the guest:
54
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
57
58Guest Data Persistence
59----------------------
60
61 Guest data persistence on non-NVDIMM requires guest userspace applications
62 to perform fsync/msync. This is different from a real nvdimm backend where
63 no additional fsync/msync is required. This is to persist guest writes in
64 host backing file which otherwise remains in host page cache and there is
65 risk of losing the data in case of power failure.
66
67 With virtio pmem device, MAP_SYNC mmap flag is not supported. This provides
68 a hint to application to perform fsync for write persistence.
69
70Limitations
71------------
72- Real nvdimm device backend is not supported.
73- virtio pmem hotunplug is not supported.
74- ACPI NVDIMM features like regions/namespaces are not supported.
75- ndctl command is not supported.
76