Re: [Qemu-devel] [PATCH] docs: add document to explain the usage of vNVDIMM

[Stefan Hajnoczi]

On Tue, Nov 08, 2016 at 08:46:14PM +0800, Haozhong Zhang wrote:
> Signed-off-by: Haozhong Zhang <address@hidden>
> Reviewed-by: Xiao Guangrong <address@hidden>
> ---
>  docs/nvdimm.txt | 124 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 124 insertions(+)
>  create mode 100644 docs/nvdimm.txt
> 
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> new file mode 100644
> index 0000000..fafca39
> --- /dev/null
> +++ b/docs/nvdimm.txt
> @@ -0,0 +1,124 @@
> +QEMU Virtual NVDIMM
> +===================
> +
> +This document explains the usage of virtual NVDIMM (vNVDIMM) feature
> +which is available since QEMU v2.6.0.
> +
> +The current QEMU only implements the persistent memory mode of vNVDIMM
> +device.

"and not the block window mode."

Explicitly naming block window mode would be useful for anyone looking
through the docs to find out whether this mode is supported or not.

> +
> +Basic Usage
> +-----------
> +
> +The storage of a vNVDIMM device in QEMU is provided by the memory
> +backend (i.e. memory-backend-file and memory-backend-ram). A simple
> +way to create a vNVDIMM device at startup time is done via the
> +following command line options:
> +
> + -machine pc,nvdimm
> + -m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE
> + -object 
> memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE
> + -device nvdimm,id=nvdimm1,memdev=mem1
> +
> +Where,
> +
> + - the "nvdimm" machine option enables vNVDIMM feature.
> +
> + - "slots=$N" should be equal to or larger than the total amount of
> +   normal RAM devices and vNVDIMM devices, e.g. $N should be >= 2 here.
> +
> + - "maxmem=$MAX_SIZE" should be equal to or larger than the total size
> +   of normal RAM devices and vNVDIMM devices, e.g. $MAX_SIZE should be
> +   >= $RAM_SIZE + $NVDIMM_SIZE here.
> +
> + - "object 
> memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE"
> +   creates a backend storage of size $NVDIMM_SIZE on a file $PATH. All
> +   accesses to the virtual NVDIMM device go to the file $PATH.
> +
> +   "share=on/off" controls the visibility of guest writes. If
> +   "share=on", then guest writes will be applied to the backend
> +   file. If another guest uses the same backend file with option
> +   "share=on", then above writes will be visible to it as well. If
> +   "share=off", then guest writes won't be applied to the backend
> +   file and thus will be invisible to other guests.
> +
> + - "device nvdimm,id=nvdimm1,memdev=mem1" creates a virtual NVDIMM
> +   device whose storage is provided by above memory backend device.
> +
> +Multiple vNVDIMM devices can be created if multiple pairs of "-object"
> +and "-device" are provided.
> +
> +For above command line options, if the guest OS has the proper NVDIMM
> +driver, it should be able to detect a NVDIMM device which is in the
> +persistent memory mode and whose size is $NVDIMM_SIZE.
> +
> +Note:
> +
> +1. Prior to QEMU v2.8.0, if memory-backend-file is used and the actual
> +   backend file size is not equal to the size given by "size" option,
> +   QEMU will truncate the backend file by ftruncate(2), which will
> +   corrupt the existing data in the backend file, especially for the
> +   shrink case.
> +
> +   QEMU v2.8.0 and later check the backend file size and the "size"
> +   option. If they do not match, QEMU will report errors and abort in
> +   order to avoid the data corruption.
> +
> +2. QEMU v2.6.0 only puts a basic alignment requirement on the "size"
> +   option of memory-backend-file, e.g. 4KB alignment on x86.  However,
> +   QEMU v.2.7.0 puts an additional alignment requirement, which may
> +   require a larger value than the basic one, e.g. 2MB on x86. This
> +   change breaks the usage of memory-backend-file that only satisfies
> +   the basic alignment.
> +
> +   QEMU v2.8.0 and later remove the additional alignment on non-s390x
> +   architectures, so the broken memory-backend-file can work again.
> +
> +Label
> +-----
> +
> +QEMU v2.7.0 and later implement the label support for vNVDIMM devices.
> +To enable label on vNVDIMM devices, users can simply add
> +"label-size=$SZ" option to "-device nvdimm", e.g.
> +
> + -device nvdimm,id=nvdimm1,memdev=mem1,label_size=128K
> +
> +Note:
> +
> +1. The minimal label size is 128KB.
> +
> +2. QEMU v2.7.0 and later store labels at the end of backend storage.
> +   If a memory backend file, which was previously used as the backend
> +   of a vNVDIMM device without labels, is now used for a vNVDIMM
> +   device with label, the data in the label area at the end of file
> +   will be inaccessible to the guest. If any useful data (e.g. the
> +   meta-data of the file system) was stored there, the latter usage
> +   may result guest data corruption (e.g. breakage of guest file
> +   system).
> +
> +Hotplug
> +-------
> +
> +QEMU v2.8.0 and later implement the hotplug support for vNVDIMM
> +devices. Similarly to the RAM hotplug, the vNVDIMM hotplug is
> +accomplished by two monitor commands "object_add" and "device_add".
> +
> +For example, the following commands add another 4GB vNVDIMM device to
> +the guest:
> +
> + (qemu) object_add 
> memory-backend-file,id=mem2,share=on,mem-path=new_nvdimm.img,size=4G
> + (qemu) device_add nvdimm,id=nvdimm2,memdev=mem2
> +
> +Note:
> +
> +1. Each hotplugged vNVDIMM device consumes one memory slot. Users
> +   should always ensure the memory option "-m ...,slots=N" specifies
> +   enough number of slots, i.e.
> +     N >= number of RAM devices +
> +          number of statically plugged vNVDIMM devices +
> +          number of hotplugged vNVDIMM devices
> +
> +2. The similar is required for the memory option "-m ...,maxmem=M", i.e.
> +     M >= size of RAM devices +
> +          size of statically plugged vNVDIMM devices +
> +          size of hotplugged vNVDIMM devices

Reviewed-by: Stefan Hajnoczi <address@hidden>

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