Re: [Qemu-devel] [PATCH v2] virtio pmem: user document

[Cornelia Huck]

On Wed, 21 Aug 2019 13:06:30 +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>
> ---
> v1->v2
>  - Fixes on text format and 'Guest Data persistence'
>    section - Cornelia
> 
>  docs/virtio-pmem.rst | 75 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 75 insertions(+)
>  create mode 100644 docs/virtio-pmem.rst
> 
> diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst
> new file mode 100644
> index 0000000000..0346e61674
> --- /dev/null
> +++ b/docs/virtio-pmem.rst
> @@ -0,0 +1,75 @@
> +
> +========================
> +QEMU virtio pmem
> +========================
> +
> + This document explains the setup and usage of virtio pmem device

s/virtio pmem device/the virtio pmem device/

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

s/The virtio pmem/The virtio pmem device/

> + regular(i.e non-NVDIMM) storage.

missing blank before '('

> +
> +Usecase
> +--------
> +
> +  Allows to bypass the guest page cache and directly use host page cache.

"Virtio pmem allows to..." ?

> +  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

missing blank before '('

> +  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 of size on a mem-path.

"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'."

Please lose the '"' (copy/paste leftover, I presume? :)

> +
> +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 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.

Only some nits from my side, otherwise looks good to me.