Backend memory object creation - query

M, Shivakumar shivakumar.m at
Thu Jan 27 14:43:01 UTC 2022

Hi Peter,

Sorry for the incomplete description of the issue, we are working on a virtualization use case and trying to automate the VM creation and management using libvirt-python.

Since Libvirt is basically XML oriented, we wanted to convert the existing tested QEMU commands into Libvirt XML for VM management.

Below is the QEMU command - its SRIOV based GPU pass-thru in our system with KVM hypervisor

qemu-system-x86_64 \

-m 4096 \

-enable-kvm \

-cpu host \

-smp cores=6 \

-drive file=/usr/share/qemu/OVMF.fd,format=raw,if=pflash \

-drive id=ubuntu_drive,if=virtio,file=/home/ubuntu/ubuntu_1.qcow2,format=qcow2,cache=none \

-usb \

-device usb-tablet \

-device virtio-vga,max_outputs=1,blob=true \

-display gtk,gl=on \

-object memory-backend-memfd,id=mem1,size=4096M \

-machine memory-backend=mem1 \

-device vfio-pci,host=0000:00:02.2

Libvirt XML (Memory backing added)
<domain type='kvm' xmlns:qemu=''>
  <memory unit="KiB">4194304</memory>
  <currentMemory unit="KiB">4194304</currentMemory>
    <source type='memfd'/>

    <type arch="x86_64" machine="pc">hvm</type>
    <boot dev="hd" />
    <boot dev="cdrom" />
    <acpi />
    <apic />
  <cpu mode='host-passthrough'>
  <clock offset="utc">
    <disk type="file" device="disk">
       <driver name="qemu" type="qcow2" />
       <source file="=/home/ubuntu/ubuntu_1.qcow2" />
       <target dev="vda" bus="virtio" />
    <serial type="pty">
      <target port="0" />
      <target type="serial" port="0">
      <alias name="serial0" />
    <console type="pty">
      <target type="serial" port="0" />
      <alias name="serial0" />
    <hostdev mode="subsystem" type="pci" managed="yes">
    <driver name='vfio'/>
              <address domain="0x0000" bus="0x00" slot="0x02" function="0x03" />
    <model type='virtio' virgl='off'/>
     <driver name='qemu'/>
    <qemu:arg value='-set'/>
    <qemu:arg value='device.video0.blob=false'/>
    <qemu:arg value='-display'/>
    <qemu:arg value='gtk,gl=on'/>

When we check the Libvirt logs, we noticed that converted XML to QEMU args are almost similar to the original
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin \
HOME=/var/lib/libvirt/qemu/domain-7-ubuntu-vm-test \
XDG_DATA_HOME=/var/lib/libvirt/qemu/domain-7-ubuntu-vm-test/.local/share \
XDG_CACHE_HOME=/var/lib/libvirt/qemu/domain-7-ubuntu-vm-test/.cache \
XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain-7-ubuntu-vm-test/.config \
DISPLAY=:1.0 \
/usr/bin/qemu-system-x86_64 \
-name guest=ubuntu-vm-test,debug-threads=on \
-S \
-object '{"qom-type":"secret","id":"masterKey0","format":"raw","file":"/var/lib/libvirt/qemu/domain-7-ubuntu-vm-test/master-key.aes"}' \
-machine pc-i440fx-6.0,usb=off,dump-guest-core=off,memory-backend=pc.ram \
-accel kvm \
-cpu host,migratable=on \
-bios /usr/share/qemu/OVMF.fd \
-m 4096 \
-object '{"qom-type":"memory-backend-memfd","id":"pc.ram","x-use-canonical-path-for-ramblock-id":false,"size":4294967296}' \
-overcommit mem-lock=off \
-smp 6,sockets=6,cores=1,threads=1 \
-uuid 25e70b21-5dc9-44ff-9e7a-026ba637f521 \
-display none \
-no-user-config \
-nodefaults \
-chardev socket,id=charmonitor,fd=32,server=on,wait=off \
-mon chardev=charmonitor,id=monitor,mode=control \
-rtc base=utc \
-no-shutdown \
-boot strict=on \
-device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 \
-blockdev '{"driver":"file","filename":"/home/ubuntu/ubuntu_1.qcow2","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}' \
-blockdev '{"node-name":"libvirt-1-format","read-only":false,"driver":"qcow2","file":"libvirt-1-storage","backing":null}' \
-device virtio-blk-pci,bus=pci.0,addr=0x3,drive=libvirt-1-format,id=virtio-disk0,bootindex=1 \
-chardev pty,id=charserial0 \
-device isa-serial,chardev=charserial0,id=serial0 \
-audiodev '{"id":"audio1","driver":"none"}' \
-device virtio-vga,id=video0,max_outputs=1,bus=pci.0,addr=0x2 \
-device vfio-pci,host=0000:00:02.3,id=hostdev0,bus=pci.0,addr=0x4 \
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5 \
-set device.video0.blob=false \
-display gtk,gl=on \
-sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
-msg timestamp=on
Our goal is to have GTK window enabled using libvirt-Python API, and we are seeing below issues with the lIbvirt but same commands working thgrough QEMU args.
[cid:image001.png at 01D813BA.46F75A10]
2022-01-22T10:49:08.685664Z qemu-system-x86_64: egl: eglGetDisplay failed
2022-01-22T10:49:08.685792Z qemu-system-x86_64: OpenGL is not supported by the display
2022-01-22 10:49:08.688+0000: shutting down, reason=failed

I assume there must be some security policies set at the Libvirt side to access the OpenGL , please help us to resolve it .



-----Original Message-----
From: Peter Krempa <pkrempa at>
Sent: Thursday, January 27, 2022 3:37 PM
To: M, Shivakumar <shivakumar.m at>
Cc: libvirt-users at; libvir-list at
Subject: Re: Backend memory object creation - query

On Thu, Jan 27, 2022 at 04:05:19 +0000, M, Shivakumar wrote:

> Hello,


note that there's no need to CC both libvir-list and libvirt-users maling lists.



> For our use-case with Libvirt we want to create the Memory backend

> object ,


> Expected QEMU args would be

> -object memory-backend-memfd,id=mem1,size=4096M

Your description is a bit vague. Do you want to use it to back the guest memory?

As by itself a memory object itself is useless without being attached somewhere

In case you want to configure qemu to back the default memory of the VM by a memfd:


<domain type='kvm'>



   <source type='memfd'/>




Results into invoking qemu with:

-machine pc-i440fx-2.9,usb=off,vmport=off,dump-guest-core=off,memory-backend=pc.ram \ ...

-object '{"qom-type":"memory-backend-memfd","id":"pc.ram","x-use-canonical-path-for-ramblock-id":false,"size":1048576000,"host-nodes":[0],"policy":"bind"}' \


> Request you to please help us to specify this arg in the libvirt XML.


> Thanks,

> Shiv

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