March 2026 - Users - libvirt List Archives

enable nested virtualization inside VM
by fc26wuqa＠studserv.uni-leipzig.de 31 Mar '26

31 Mar '26

Hello, I want to set up a NAT network inside a VM (so far as I understand, the name for that is nested virtualization). Unfortunately I only have Shell Access to the VM, and am not hosting it. I installed debian. The output of `uname -a`: Linux kvm1 6.12.74+deb13+1-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.74-2 (2026-03-08) x86_64 GNU/Linux When I run `kvm-ok` it says "INFO: Your CPU does not support KVM extensions KVM acceleration can NOT be used". When I try to load `kvm_intel` into the kernel (even with nested=1 option), it says "modprobe: ERROR: could not insert 'kvm_intel': Operation not supported". I can't start a network, there is no `/dev/kvm` device. I installed libvirt with `apt install --no-install-recommends qemu-system libvirt-clients libvirt-daemon-system`. Is there a way to enable libvirt support? For me, or the admins that host the VMs? Thank you.

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double startup of sessioned vm causes user systemd service to fail
by daggs 11 Mar '26

11 Mar '26

Greetings, I'm running 3 vms in session mode on arch linux, I need each vm to startup on boot. libvirt version is 12.1.0. from what I understand, libvirt-guests lacks session support so I went to look and found that I can use per user systemd service, using this doc: https://wiki.archlinux.org/title/Systemd/User I defined each user to linger, created a service file and enabled it for each user. I've rebooted the machine and found out that not all the vms started, some of them have failed, each boot the outcome differs but usually there is atleast one vm which doesn't startup. I've check the status of the service and found that in the cases where the boot failed, the error was that the vm is already active, however virsh list shows ihe vm is off. none of the vms is marked as autostart. how can I find out who is the other source that starts up the vms? I assume it is something internal of libvirt. ideally, if possible, i'd like to use something from within libvirt or at least stop whatever autoboots the vms. Thanks, Dagg

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Writeback vs default cache modes
by João Jandre 04 Mar '26

04 Mar '26

Hi, all I've been reading the QEMU documentation, the Libvirt documentation, as well as doing some testing and code investigation regarding disk cache modes. In the Libvirt documentation, it is stated that the disk driver cache modes accepted are "default", "none", "writethrough", "writeback", "directsync" and "unsafe". Analyzing the code, we can see that this is somewhat reflected on the cache modes that QEMU documents on the -drive option (https://qemu-project.gitlab.io/qemu/system/invocation.html) https://github.com/libvirt/libvirt/blob/04c1f458313e9001f5a804a898408e1f498… However, the default option (when we do not define the "cache" option) is different from other options and does not seem to map to any option on the Qemu side. I see that, from testing and analyzing the code, if the default (not defining the "cache" options) is used, cache.direct and cache.no-flush are not informed; also, the write-cache option is not informed. Just for comparison, here is a disk defined with cache=writeback: XML: |<disk type='file' device='disk'> <driver name='qemu' type='qcow2' cache='writeback'/> <source file='/mnt/a66f48f5-13c6-3676-b320-a15d32bbf32f/c0e925a9-6697-4ee9-8abf-d36c7a95cea4' index='2'/> <backingStore type='file' index='3'> <format type='qcow2'/> <source file='/mnt/a66f48f5-13c6-3676-b320-a15d32bbf32f/0db52809-9f33-43ac-8151-5452e9dee330'/> <backingStore/> </backingStore> <target dev='vda' bus='virtio'/> <serial>c0e925a966974ee98abf</serial> <alias name='virtio-disk0'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/> </disk> | Generated QEMU command: |-blockdev {"driver":"file","filename":"/mnt/a66f48f5-13c6-3676-b320-a15d32bbf32f/c0e925a9-6697-4ee9-8abf-d36c7a95cea4","node-name":"libvirt-2-storage","cache":{"direct":false,"no-flush":false},"auto-read-only":true,"discard":"unmap"} -blockdev {"node-name":"libvirt-2-format","read-only":false,"cache":{"direct":false,"no-flush":false},"driver":"qcow2","file":"libvirt-2-storage","backing":"libvirt-3-format"} -device virtio-blk-pci,bus=pci.0,addr=0x5,drive=libvirt-2-format,id=virtio-disk0,bootindex=2,write-cache=on,serial=c0e925a966974ee98abf | And here is a disk that does not have the cache option: XML: |<disk type='file' device='disk'> <driver name='qemu' type='qcow2'/> <source file='/mnt/b59eff0f-5b97-37ed-a513-9e1983d1d19b/b430296b-0924-412e-a7b1-ddc3d4c90f83' index='2'/> <backingStore/> <target dev='vdc' bus='virtio'/> <serial>b430296b0924412ea7b1</serial> <alias name='virtio-disk2'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/> </disk> | Generated QEMU command: |blockdev {"driver":"file","filename":"/mnt/b59eff0f-5b97-37ed-a513-9e1983d1d19b/b430296b-0924-412e-a7b1-ddc3d4c90f83","node-name":"libvirt-2-storage","auto-read-only":true,"discard":"unmap"} -blockdev {"node-name":"libvirt-2-format","read-only":false,"driver":"qcow2","file":"libvirt-2-storage","backing":null} -device virtio-blk-pci,bus=pci.0,addr=0x8,drive=libvirt-2-format,id=virtio-disk2,serial=b430296b0924412ea7b1 | The QEMU documentation states in the -drive option that the default cache mode is |Writeback| (see https://qemu-project.gitlab.io/qemu/system/invocation.html) so I expected that setting |Writeback| or |Default| as the cache mode on Libvirt would also be the same. I ran some performance tests using both options to try and verify if they were the same, but they are clearly different. For each of the configuration profiles below, I created a VM and ran some fio tests: Combination Disk cache mode Disk Controller IO thread IO policy hds0i default virtio-scsi 0 iouring hds0t default virtio-scsi 0 threads hds1i default virtio-scsi 1 iouring hds1t default virtio-scsi 1 threads hdv0i default virtio 0 iouring hdv0t default virtio 0 threads hdv1i default virtio 1 iouring hdv1t default virtio 1 threads wbs0i writeback virtio-scsi 0 iouring wbs0t writeback virtio-scsi 0 threads wbs1i writeback virtio-scsi 1 iouring wbs1t writeback virtio-scsi 1 threads wbv0i writeback virtio 0 iouring wbv0t writeback virtio 0 threads wbv1i writeback virtio 1 iouring wbv1t writeback virtio 1 threads The fio test results are in a html attached to the email. I would like to understand what the practical difference is between not informing the cache and any other option. I can see that the disk is defined in different way, but just by analyzing the code I was not able to understand what that leads to; also, looking at the performance tests, we clearly see a difference in the results. Would anyone be able to explain what is the expected behavior when the disk is defined in such a way? By the way, I'm using Libvirt 8.0.0 and Qemu 6.2.0. Best regards, João Jandre

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