Re: [Qemu-devel] [PATCH] virtio pmem: user document

[Cornelia Huck]

On Tue, 30 Jul 2019 12:16:57 +0530
Pankaj Gupta <address@hidden> wrote:

> 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 <address@hidden>
> ---
>  docs/virtio-pmem.txt | 65 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
>  create mode 100644 docs/virtio-pmem.txt
> 
> diff --git a/docs/virtio-pmem.txt b/docs/virtio-pmem.txt

Maybe make this ReST from the start? Should be trivial enough.

> new file mode 100644
> index 0000000000..fc61eebb20
> --- /dev/null
> +++ b/docs/virtio-pmem.txt
> @@ -0,0 +1,65 @@
> +
> +QEMU virtio pmem
> +===================
> +
> + This document explains the usage of virtio pmem device 

"setup and usage" ?

> + which is available since QEMU v4.1.0.
> +
> + The virtio pmem is paravirtualized persistent memory device

"The virtio pmem device is a paravirtualized..."

> + 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

s/as host/,as the host/

> +  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

s/regular(non-NVDIMM)/regular (i.e. non-NVDIMM)/ ?
s/persists/persist/

> +  the guest writes as there are no defined semantecs in the device 
> specification.

s/semantecs/semantics/

> +  With virtio pmem device, guest write persistence on non-NVDIMM storage is
> +  supported.

"The virtio pmem device provides a way to support guest write
persistence on non-NVDIMM storage." ?

> +
> +virtio pmem usage
> +-----------------
> +  virtio pmem device is created with a memory-backend-file with the below
> +  options:

"A virtio pmem device backed by a memory-backend-file can be created on
the QEMU command line as in the following example:" ?

> +
> +  -machine pc -m 8G,slots=$N,maxmem=$MAX_SIZE

I'm not sure you should explicitly specify the machine type in this
example. I think it is fine to say that something is only supported on
a subset of machine types, but it should not make its way into an
example on how to configure a device and its backing.

Also, maybe fill in more concrete values here? Or split it into a part
specifying the syntax (where I'd use <max_size> instead of $MAX_SIZE
etc.), and a more concrete example?

> +  -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.

"a virtio pmem PCI 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".

Hm... what about the following instead:

"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 application to 

s/application/applications/ ?

> +perform fsync/msync. This is different than real nvdimm backend where no 
> additional

s/than/from a/ ?

> +fsync/msync is required for data persistence.

Should we be a bit more verbose on what which guest applications are
supposed to do? I.e., how do they know they need to do fsync/msync,
when should they do it, and what are the consequences if they don't?

> +
> +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.