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

[Pankaj Gupta <pagupta@redhat.com>]

> On Tue, 30 Jul 2019 12:16:57 +0530
> Pankaj Gupta <pagupta@redhat.com> 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 <pagupta@redhat.com>
> > ---
> >  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.

o.k

> 
> > 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" ?

o.k

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

sure.

> 
> > + 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/

sure

> 
> > +  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/

yes, to both.

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

ah...spell checker :(

> 
> > +  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:" ?

o.k

> 
> > +
> > +  -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.

o.k

> 
> 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?

o.k

> 
> > +  -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" ?

o.k

> 
> > +
> > +  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'."

o.k

> 
> > +
> > +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/ ?

yes.

> 
> > +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?

o.k. 

> 
> > +
> > +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.

Thank you for the review.

Best regards,
Pankaj
> 
> 
>