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author | Peter Maydell <peter.maydell@linaro.org> | 2019-09-17 16:24:08 +0100 |
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committer | Peter Maydell <peter.maydell@linaro.org> | 2019-09-17 16:24:08 +0100 |
commit | f39641125996550b76f9ebbfe03430aecc6473c4 (patch) | |
tree | 3ae55845e6681349c6c1c11468c2b763a4b5a482 /docs/virtio-pmem.rst | |
parent | f8c3db33a5e863291182f8862ddf81618a7c6194 (diff) | |
parent | 44e687a4d9ab327761e221844ced7dc9c23350a5 (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>
Diffstat (limited to 'docs/virtio-pmem.rst')
-rw-r--r-- | docs/virtio-pmem.rst | 75 |
1 files changed, 75 insertions, 0 deletions
diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst new file mode 100644 index 0000000000..e77881b26f --- /dev/null +++ b/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. |