[libvirt] Designing NVDIMM & memory-backend-file

[Michal Privoznik]

Dear list,

I'd like to fix the following bug [1]. Long story short, the only way
how to have a domain use memory-backend-file object is to configure
hugepages. Either for whole domain under <memoryBacking/> element, or in
<memory/> device:

  <memoryBacking>
    <hugepages>
      <page size='2048' unit='KiB' nodeset='1'/>
    </hugepages>
  </memoryBacking>

-object memory-backend-file,id=ram-node1,prealloc=yes,\
mem-path=/dev/hugepages2M/libvirt/qemu,size=1073741824,host-nodes=0-3,\
policy=bind \
-numa node,nodeid=1,cpus=1,memdev=ram-node1 \


    <memory model='dimm'>
      <source>
        <nodemask>1-3</nodemask>
        <pagesize unit='KiB'>2048</pagesize>
      </source>
      <target>
        <size unit='KiB'>524287</size>
        <node>0</node>
      </target>
    </memory>

-object memory-backend-file,id=memdimm1,prealloc=yes,\
mem-path=/dev/hugepages2M/libvirt/qemu,size=536870912,host-nodes=1-3,policy=bind
\
-device pc-dimm,node=0,memdev=memdimm1,id=dimm1 \

Now, there's a request in the BZ to allow applications to use the
memory-backend-file but let them use different backend, well different
path for backing the memory, e.g. shm which is usually mounted at
/dev/shm. And I'd like to consult my idea before I dig deep in the patches.

My idea is to extend <memory/> device we have, more precisely <source/>
element to allow something like this:

    <memory model='dimm'>
      <source>
        <path>/dev/shm</path>
      </source>
      <target/>
    </memory>

This way we can allow users to pass an arbitrary path to the
memory-backend-file. Also the amount of code needing change would be
fairly small O:-)

And while I'll be working on this, I want to take a look at NVDIMM
feature that qemu introduced recently [2]. The way I understand it, it
is very similar to my problem from above. The only difference (for the
consistence) would be that the memory model would be something else than
'dimm' ('nvdimm' perhaps?)

    <memory model='nvdimm'>
      <source>
        <path>/tmp/nvdimm1</path>
      </source>
      <target>
        <size unit='GiB'>10</size>
      </target>
    </memory>


-machine pc,nvdimm -m 8G,maxmem=100G,slots=100 \
-object memory-backend-file,id=mem1,share,mem-path=/tmp/nvdimm1,size=10G \
-device nvdimm,memdev=mem1,id=nv1


Please, let me know what do you think of this. Thank you for saving me
from doing couple of useless iterations of patches getting the design
right :-)

Michal


1: https://bugzilla.redhat.com/show_bug.cgi?id=1214369

2: http://lists.nongnu.org/archive/html/qemu-devel/2016-06/msg07068.html