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266 lines
10 KiB
Plaintext
266 lines
10 KiB
Plaintext
QEMU Virtual NVDIMM
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===================
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This document explains the usage of virtual NVDIMM (vNVDIMM) feature
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which is available since QEMU v2.6.0.
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The current QEMU only implements the persistent memory mode of vNVDIMM
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device and not the block window mode.
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Basic Usage
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-----------
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The storage of a vNVDIMM device in QEMU is provided by the memory
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backend (i.e. memory-backend-file and memory-backend-ram). A simple
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way to create a vNVDIMM device at startup time is done via the
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following command line options:
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-machine pc,nvdimm=on
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-m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE
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-object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE,readonly=off
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-device nvdimm,id=nvdimm1,memdev=mem1,unarmed=off
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Where,
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- the "nvdimm" machine option enables vNVDIMM feature.
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- "slots=$N" should be equal to or larger than the total amount of
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normal RAM devices and vNVDIMM devices, e.g. $N should be >= 2 here.
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- "maxmem=$MAX_SIZE" should be equal to or larger than the total size
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of normal RAM devices and vNVDIMM devices, e.g. $MAX_SIZE should be
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>= $RAM_SIZE + $NVDIMM_SIZE here.
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- "object memory-backend-file,id=mem1,share=on,mem-path=$PATH,
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size=$NVDIMM_SIZE,readonly=off" creates a backend storage of size
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$NVDIMM_SIZE on a file $PATH. All accesses to the virtual NVDIMM device go
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to the file $PATH.
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"share=on/off" controls the visibility of guest writes. If
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"share=on", then guest writes will be applied to the backend
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file. If another guest uses the same backend file with option
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"share=on", then above writes will be visible to it as well. If
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"share=off", then guest writes won't be applied to the backend
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file and thus will be invisible to other guests.
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"readonly=on/off" controls whether the file $PATH is opened read-only or
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read/write (default).
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- "device nvdimm,id=nvdimm1,memdev=mem1,unarmed=off" creates a read/write
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virtual NVDIMM device whose storage is provided by above memory backend
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device.
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"unarmed" controls the ACPI NFIT NVDIMM Region Mapping Structure "NVDIMM
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State Flags" Bit 3 indicating that the device is "unarmed" and cannot accept
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persistent writes. Linux guest drivers set the device to read-only when this
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bit is present. Set unarmed to on when the memdev has readonly=on.
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Multiple vNVDIMM devices can be created if multiple pairs of "-object"
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and "-device" are provided.
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For above command line options, if the guest OS has the proper NVDIMM
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driver (e.g. "CONFIG_ACPI_NFIT=y" under Linux), it should be able to
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detect a NVDIMM device which is in the persistent memory mode and whose
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size is $NVDIMM_SIZE.
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Note:
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1. Prior to QEMU v2.8.0, if memory-backend-file is used and the actual
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backend file size is not equal to the size given by "size" option,
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QEMU will truncate the backend file by ftruncate(2), which will
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corrupt the existing data in the backend file, especially for the
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shrink case.
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QEMU v2.8.0 and later check the backend file size and the "size"
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option. If they do not match, QEMU will report errors and abort in
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order to avoid the data corruption.
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2. QEMU v2.6.0 only puts a basic alignment requirement on the "size"
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option of memory-backend-file, e.g. 4KB alignment on x86. However,
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QEMU v.2.7.0 puts an additional alignment requirement, which may
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require a larger value than the basic one, e.g. 2MB on x86. This
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change breaks the usage of memory-backend-file that only satisfies
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the basic alignment.
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QEMU v2.8.0 and later remove the additional alignment on non-s390x
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architectures, so the broken memory-backend-file can work again.
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Label
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-----
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QEMU v2.7.0 and later implement the label support for vNVDIMM devices.
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To enable label on vNVDIMM devices, users can simply add
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"label-size=$SZ" option to "-device nvdimm", e.g.
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-device nvdimm,id=nvdimm1,memdev=mem1,label-size=128K
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Note:
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1. The minimal label size is 128KB.
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2. QEMU v2.7.0 and later store labels at the end of backend storage.
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If a memory backend file, which was previously used as the backend
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of a vNVDIMM device without labels, is now used for a vNVDIMM
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device with label, the data in the label area at the end of file
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will be inaccessible to the guest. If any useful data (e.g. the
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meta-data of the file system) was stored there, the latter usage
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may result guest data corruption (e.g. breakage of guest file
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system).
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Hotplug
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-------
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QEMU v2.8.0 and later implement the hotplug support for vNVDIMM
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devices. Similarly to the RAM hotplug, the vNVDIMM hotplug is
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accomplished by two monitor commands "object_add" and "device_add".
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For example, the following commands add another 4GB vNVDIMM device to
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the guest:
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(qemu) object_add memory-backend-file,id=mem2,share=on,mem-path=new_nvdimm.img,size=4G
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(qemu) device_add nvdimm,id=nvdimm2,memdev=mem2
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Note:
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1. Each hotplugged vNVDIMM device consumes one memory slot. Users
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should always ensure the memory option "-m ...,slots=N" specifies
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enough number of slots, i.e.
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N >= number of RAM devices +
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number of statically plugged vNVDIMM devices +
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number of hotplugged vNVDIMM devices
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2. The similar is required for the memory option "-m ...,maxmem=M", i.e.
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M >= size of RAM devices +
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size of statically plugged vNVDIMM devices +
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size of hotplugged vNVDIMM devices
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Alignment
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---------
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QEMU uses mmap(2) to maps vNVDIMM backends and aligns the mapping
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address to the page size (getpagesize(2)) by default. However, some
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types of backends may require an alignment different than the page
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size. In that case, QEMU v2.12.0 and later provide 'align' option to
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memory-backend-file to allow users to specify the proper alignment.
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For device dax (e.g., /dev/dax0.0), this alignment needs to match the
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alignment requirement of the device dax. The NUM of 'align=NUM' option
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must be larger than or equal to the 'align' of device dax.
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We can use one of the following commands to show the 'align' of device dax.
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ndctl list -X
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daxctl list -R
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In order to get the proper 'align' of device dax, you need to install
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the library 'libdaxctl'.
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For example, device dax require the 2 MB alignment, so we can use
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following QEMU command line options to use it (/dev/dax0.0) as the
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backend of vNVDIMM:
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-object memory-backend-file,id=mem1,share=on,mem-path=/dev/dax0.0,size=4G,align=2M
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-device nvdimm,id=nvdimm1,memdev=mem1
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Guest Data Persistence
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----------------------
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Though QEMU supports multiple types of vNVDIMM backends on Linux,
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the only backend that can guarantee the guest write persistence is:
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A. DAX device (e.g., /dev/dax0.0, ) or
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B. DAX file(mounted with dax option)
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When using B (A file supporting direct mapping of persistent memory)
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as a backend, write persistence is guaranteed if the host kernel has
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support for the MAP_SYNC flag in the mmap system call (available
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since Linux 4.15 and on certain distro kernels) and additionally
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both 'pmem' and 'share' flags are set to 'on' on the backend.
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If these conditions are not satisfied i.e. if either 'pmem' or 'share'
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are not set, if the backend file does not support DAX or if MAP_SYNC
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is not supported by the host kernel, write persistence is not
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guaranteed after a system crash. For compatibility reasons, these
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conditions are ignored if not satisfied. Currently, no way is
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provided to test for them.
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For more details, please reference mmap(2) man page:
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http://man7.org/linux/man-pages/man2/mmap.2.html.
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When using other types of backends, it's suggested to set 'unarmed'
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option of '-device nvdimm' to 'on', which sets the unarmed flag of the
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guest NVDIMM region mapping structure. This unarmed flag indicates
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guest software that this vNVDIMM device contains a region that cannot
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accept persistent writes. In result, for example, the guest Linux
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NVDIMM driver, marks such vNVDIMM device as read-only.
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Backend File Setup Example
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--------------------------
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Here are two examples showing how to setup these persistent backends on
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linux using the tool ndctl [3].
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A. DAX device
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Use the following command to set up /dev/dax0.0 so that the entirety of
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namespace0.0 can be exposed as an emulated NVDIMM to the guest:
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ndctl create-namespace -f -e namespace0.0 -m devdax
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The /dev/dax0.0 could be used directly in "mem-path" option.
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B. DAX file
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Individual files on a DAX host file system can be exposed as emulated
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NVDIMMS. First an fsdax block device is created, partitioned, and then
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mounted with the "dax" mount option:
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ndctl create-namespace -f -e namespace0.0 -m fsdax
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(partition /dev/pmem0 with name pmem0p1)
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mount -o dax /dev/pmem0p1 /mnt
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(create or copy a disk image file with qemu-img(1), cp(1), or dd(1)
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in /mnt)
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Then the new file in /mnt could be used in "mem-path" option.
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NVDIMM Persistence
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------------------
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ACPI 6.2 Errata A added support for a new Platform Capabilities Structure
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which allows the platform to communicate what features it supports related to
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NVDIMM data persistence. Users can provide a persistence value to a guest via
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the optional "nvdimm-persistence" machine command line option:
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-machine pc,accel=kvm,nvdimm,nvdimm-persistence=cpu
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There are currently two valid values for this option:
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"mem-ctrl" - The platform supports flushing dirty data from the memory
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controller to the NVDIMMs in the event of power loss.
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"cpu" - The platform supports flushing dirty data from the CPU cache to
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the NVDIMMs in the event of power loss. This implies that the
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platform also supports flushing dirty data through the memory
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controller on power loss.
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If the vNVDIMM backend is in host persistent memory that can be accessed in
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SNIA NVM Programming Model [1] (e.g., Intel NVDIMM), it's suggested to set
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the 'pmem' option of memory-backend-file to 'on'. When 'pmem' is 'on' and QEMU
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is built with libpmem [2] support (configured with --enable-libpmem), QEMU
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will take necessary operations to guarantee the persistence of its own writes
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to the vNVDIMM backend(e.g., in vNVDIMM label emulation and live migration).
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If 'pmem' is 'on' while there is no libpmem support, qemu will exit and report
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a "lack of libpmem support" message to ensure the persistence is available.
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For example, if we want to ensure the persistence for some backend file,
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use the QEMU command line:
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-object memory-backend-file,id=nv_mem,mem-path=/XXX/yyy,size=4G,pmem=on
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References
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----------
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[1] NVM Programming Model (NPM)
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Version 1.2
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https://www.snia.org/sites/default/files/technical_work/final/NVMProgrammingModel_v1.2.pdf
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[2] Persistent Memory Development Kit (PMDK), formerly known as NVML project, home page:
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http://pmem.io/pmdk/
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[3] ndctl-create-namespace - provision or reconfigure a namespace
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http://pmem.io/ndctl/ndctl-create-namespace.html
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