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authorPeter Maydell <peter.maydell@linaro.org>2019-09-17 16:24:08 +0100
committerPeter Maydell <peter.maydell@linaro.org>2019-09-17 16:24:08 +0100
commitf39641125996550b76f9ebbfe03430aecc6473c4 (patch)
tree3ae55845e6681349c6c1c11468c2b763a4b5a482 /docs/virtio-pmem.rst
parentf8c3db33a5e863291182f8862ddf81618a7c6194 (diff)
parent44e687a4d9ab327761e221844ced7dc9c23350a5 (diff)
Merge remote-tracking branch 'remotes/mst/tags/for_upstream' into staging
virtio,vhost,pc: features, fixes, cleanups. Virtio 1.0 support for virtio-mmio. Misc fixes, cleanups. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> # gpg: Signature made Mon 16 Sep 2019 16:17:30 BST # gpg: using RSA key 281F0DB8D28D5469 # gpg: Good signature from "Michael S. Tsirkin <mst@kernel.org>" [full] # gpg: aka "Michael S. Tsirkin <mst@redhat.com>" [full] # Primary key fingerprint: 0270 606B 6F3C DF3D 0B17 0970 C350 3912 AFBE 8E67 # Subkey fingerprint: 5D09 FD08 71C8 F85B 94CA 8A0D 281F 0DB8 D28D 5469 * remotes/mst/tags/for_upstream: virtio-mmio: implement modern (v2) personality (virtio-1) virtio pmem: user document intel_iommu: Remove the caching-mode check during flag change pc/q35: Disallow vfio-pci hotplug without VT-d caching mode qdev/machine: Introduce hotplug_allowed hook intel_iommu: Sanity check vfio-pci config on machine init done backends/vhost-user.c: prevent using uninitialized vqs vhost-user-blk: prevent using uninitialized vqs docs/nvdimm: add example on persistent backend setup MAINTAINERS: update virtio-rng and virtio-serial maintainer Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
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+
+========================
+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.