virtio pmem: user document

Commit Message

Pankaj Gupta July 30, 2019, 6:46 a.m. UTC

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 <[e-mail]>
---
 docs/virtio-pmem.txt | 65 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)
 create mode 100644 docs/virtio-pmem.txt

Patch

diff --git a/docs/virtio-pmem.txt b/docs/virtio-pmem.txt
new file mode 100644
index 0000000000..fc61eebb20
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+++ b/docs/virtio-pmem.txt
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+
+QEMU virtio pmem
+===================
+
+ This document explains the usage of virtio pmem device 
+ which is available since QEMU v4.1.0.
+
+ The virtio pmem is paravirtualized persistent memory device
+ on regular(non-NVDIMM) storage. 
+
+Usecase
+--------
+  Allows to bypass the guest page cache and directly use host page cache.
+  This reduces guest memory footprint as 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(non-NVDIMM) host storage does not persists
+  the guest writes as there are no defined semantecs in the device specification.
+  With virtio pmem device, guest write persistence on non-NVDIMM storage is
+  supported.
+
+virtio pmem usage
+-----------------
+  virtio pmem device is created with a memory-backend-file with the below
+  options:
+
+  -machine pc -m 8G,slots=$N,maxmem=$MAX_SIZE
+  -object memory-backend-file,id=mem1,share,mem-path=$PATH,size=$SIZE
+  -device virtio-pmem-pci,memdev=mem1,id=nv1
+
+   where:
+   - "object memory-backend-file,id=mem1,share,mem-path=$PATH,size=$VIRTIO_PMEM_SIZE"
+     creates a backend storage of size $SIZE on a file $PATH. All
+     accesses to the virtio pmem device go to the file $PATH.
+
+   - "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem
+     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
+-------
+Accomplished by two monitor commands "object_add" and "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 application to 
+perform fsync/msync. This is different than real nvdimm backend where no additional
+fsync/msync is required for data 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.