[libvirt] adding a new libvirt xml element for File Descriptor backed memory for use with vhost-user

Daniel P. Berrange berrange at redhat.com
Wed May 11 09:43:31 UTC 2016

On Tue, May 10, 2016 at 09:01:20PM +0000, Mooney, Sean K wrote:
> Today the only native  way to request Libvirt to create a vm who's memory is backed by a  memdev which can be
> Accessed via a file descriptor is to request hugepage backed memory. This requires the operator to
> Manage and configure hugepage on each of their compute hosts and take special care to ensure that vms are
> Not placed on host were vhost-user interface are used if they do not request hugepages.
> Today it is possible to use Libvirt to spawn a vm without hugepage memory and a file descriptor backed memdev
> Via the use of the qemu:commandline element.
>   <qemu:commandline>
>     <qemu:arg value='-object'/>
>     <qemu:arg value='memory-backend-file,id=mem,size=1024M,mem-path=/var/lib/libvirt/qemu,share=on'/>
>     <qemu:arg value='-numa'/>
>     <qemu:arg value='node,memdev=mem'/>
>     <qemu:arg value='-mem-prealloc'/>
>   </qemu:commandline>
> I created a proof of concept patch to nova to demonstrate that this works however to support this usecase in
> Nova a new xml element is required.
> https://review.openstack.org/#/c/309565/1
> I would like to propose the introduction of  a new subelemnt to the memorybacking element to request file discrptro backed memory
> <memoryBacking>
>    <filedescriptor size_mb="1024" path="/var/lib/libvirt/qemu" prealloc="true" shared="on" />
>  </memoryBacking>

Specifying a size is not required - we already know how big memory must
be for the guest.

We already have a memAccess='shared' attribute against the <numa>
element that is used to determine if the underlying memory should
be setup as shared.  We could define a further element that lets
us control memory access mode for guests without NUMA topology

     <access mode="shared"/>

For huge pages it seems we unconditionally pass --mem-prealloc. I'm thinking
we could perhaps make that configurable via an element

     <allocation mode="immediate|ondemand"/>

to control use of -mem-prealloc or not.

So all that remains is a way to request file based backing of RAM. As with
huge pages, I think we should hide the actual path from the user. We should
just use /dev/shm as the backing for non-hugepage RAM. For this we could
define something like

       <source type="file|anonymous"/>

Putting that all together, to get what you want we'd have

       <source type="file"/>
       <access mode="shared"/>
       <allocation mode="immediate"/>

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