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libvirt access control
by Nikolaus Rath 05 Dec '25

05 Dec '25

Hello, I'm puzzled by how libvirtd manages access control. As far as I can tell, on my Fedora system it runs qemu and virtiofsd instances as user *qemu*. Yet, the qemu process apparently has read/write access to storage devices in /dev that only root has rw access for. Similarly, virtiofsd is able to write into shared directories that the qemu user does not have access to (confirmed by switching to qemu user with sudo). What is libvirtd (or virt-manager) doing to make this work? Then, as far as I can tell, there's lots of sockets with different permissions created by systemd in /run/libvirt (e.g. libvirt-admin-sock, libvirt-sock, libvirt-sock-ro). But since they're all passed to libvirtd as open file descriptors, I don't understand how they can give different levels of access (since libvirtd has no way of knowing which socket was used). What am I missing? Best, -Nikolaus

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virDomainBlockCommit for offline domains
by melanie witt 13 Nov '25

13 Nov '25

Hi, I am trying to find info about whether block commit of an offline domain is possible? Historically I know it was not supported and I wanted to confirm whether that is still the case. I did not notice mention about the limitation in the libvirt domain API docs or in the virsh docs. I'm investigating in the context of OpenStack Nova -- we have had a feature request brought back up recently to be able to delete an "intermediary" image in a backing chain while the VM is shutdown. And from what I understand, we would need to be able to call virDomainBlockCommit for a domain that is not running [1] as part of it. So I'm just looking to learn about the current state of support. Thanks for any information, -melwitt [1] https://bugs.launchpad.net/nova/+bug/1919487

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Linux bridge VLANs
by Ian Pilcher 13 Nov '25

13 Nov '25

Do recent versions of libvirt support configuring VLANs (truck and access ports) on Linux bridges? I've been using OVS bridges for a long time (because Linux bridges didn't even do VLANs when I started). Linux bridges should theoretically be a bit simpler, but I can't tell from the libvirt documentation if libvirt ever gained the ability to do VLANs with Linux bridges. -- ======================================================================== Google Where SkyNet meets Idiocracy ========================================================================

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Re: Compact backing chain (sparsify + compress)
by Markus Armbruster 10 Nov '25

10 Nov '25

Adding cc:s. Christopher Pereira <kripper(a)imatronix.cl> writes: > Hi, > > I would like to revisit this old thread from 2016 with a special use case that I believe should be a standard `virsh` command: > https://lists.gnu.org/archive/html/qemu-devel/2016-12/msg03571.html > > **Summary:** > > Given this QEMU backing chain: > `base <- snap1 <- snap2 <- snap3 (active)` > > We want to merge `base <- snap1 <- snap2` into a new snapshot `collapsed-base` that is: > 1. Sparsified (`virt-sparsify`) > 2. Compressed > > The resulting backing chain would be: > `collapsed-base <- snap3 (active)` > > **Motivation:** > > - We perform daily backup snapshots and never modify existing files (too dangerous). We only rebase. > - We collapse older chains into a new `collapsed-base` snapshot to limit chain size and avoid performance degradation. > > We have been doing this successfully for over 10 years using: > > - `qemu-img convert` > - `virt-sparsify` > - `virsh save` > - `qemu-img rebase` > - `virsh resume` > > **Problems:** > > - There is a small downtime due to `virsh save`/`resume`. > - In recent QEMU versions, `virsh` adds a `backingStore` tag to the XML even when using the `--no-metadata` option. This causes inconsistencies after `qemu-img rebase`. > > We didn’t use QMP because it didn’t support sparsify + compression in the past. > > **Questions:** > > - Is there now a better way to achieve this? > - Could this feature be implemented or supported directly in `virsh`? > > In my opinion, this would be the ideal backup solution: we could travel in time, sync immutable snapshots to a remote backup server, and maintain performance.

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Co-stop with non-VMware virtualization?
by Troels Arvin 05 Nov '25

05 Nov '25

Hello, How important is it to consider "co-stop" with non-VMWare virtualization such as KVM? I haven't been able to find anything about this for KVM, but there's some material about it for VMWare (albeit typically rather old material). Based on VMWare material, by "co-stop" I mean the following situation: A VM called "x" has been assigned quite a few vCPUs, e.g. 10, on a hypervisor with e.g. 20 physical cores. The hypervisor is hosting many other VMs, and the many VMs take turn running on the hardware. Now, if it's often the case that, e.g., only 7 physical threads are available when it's x's turn to run, the hypervisor needs to postpone running x till all 10 cores can be allocated at the same time. During this waiting time, x is stopped (co-stop). Is my understanding correct? Or will KVM allow x to run on (e.g.) 7 cores, even though the VM thinks it has 10 vCPUs available? It's my understanding that with KVM, the co-stopped situation is reflected in the VM's "steal time" metric in a tool like "top". Does hyperthreading in either the hypervisor and/or the guest impact the risk of co-stop? -- Regards, Troels Arvin

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[FOSDEM] Call for participation: Virtualization and Cloud infrastructure Room at FOSDEM 2026
by Piotr Kliczewski 28 Oct '25

28 Oct '25

We are excited to announce that the call for proposals is now open for the Virtualization and Cloud infrastructure devroom at the upcoming FOSDEM 2026, to be hosted on 31st of January. This devroom is a collaborative effort, and is organized by dedicated folks from projects such as OpenStack, Xen Project, KubeVirt, QEMU, KVM, and Foreman. We invite everyone involved in these fields to submit your proposals by 5th December 2025. About the Devroom The Virtualization & IaaS devroom will feature session topics such as open source hypervisors or virtual machine managers such as Xen Project, KVM, bhyve and VirtualBox as well as Infrastructure-as-a-Service projects such as KubeVirt, Apache CloudStack, OpenStack, QEMU and OpenNebula. This devroom will host presentations that focus on topics of shared interest, such as KVM, libvirt, shared storage, virtualized networking, cloud security, clustering and high availability, interfacing with multiple hypervisors, hyperconverged deployments and scaling across hundreds or thousands of servers. Presentations in this devroom will be aimed at developers working on these platforms who are looking to collaborate and improve shared infrastructure or solve common problems. We seek topics that encourage dialog between projects and continued work post-FOSDEM. Important Dates Submission deadline: 5th December 2025 Acceptance notifications: 10th December 2025 Final schedule announcement: 15th of December 2025 Devroom: 31st of January Submit Your Proposal During submission [1] please make sure to select Virtualization and Cloud infrastructure from the Track list. Please provide a meaningful abstract and description of your proposed session. Submission Guidelines We expect more proposals than we can possibly accept, so it is vitally important that you submit your proposal on or before the deadline. Late submissions are unlikely to be considered. All presentation slots are 30 minutes, with 20 minutes planned for presentations, and 10 minutes for Q&A. All presentations will be recorded and made available under Creative Commons licenses. In the Submission notes field, please indicate that you agree that your presentation will be licensed under the CC-By-SA-4.0 or CC-By-4.0 license and that you agree to have your presentation recorded. For example: "If my presentation is accepted for FOSDEM, I hereby agree to license all recordings, slides, and other associated materials under the Creative Commons Attribution Share-Alike 4.0 International License. Sincerely, Piotr Kliczewski and Tal Nisan In the Submission notes field, please also confirm that if your talk is accepted, you will be able to attend FOSDEM and deliver your presentation. We will not consider proposals from prospective speakers who are unsure whether they will be able to secure funds for travel and lodging to attend FOSDEM. (Sadly, we are not able to offer travel funding for prospective speakers.) Code of Conduct Following the release of the updated code of conduct for FOSDEM[2], we'd like to remind all speakers and attendees that all of the presentations and discussions in our devroom are held under the guidelines set in the CoC and we expect attendees, speakers, and volunteers to follow the CoC at all times. If you submit a proposal and it is accepted, you will be required to confirm that you accept the FOSDEM CoC. If you have any questions about the CoC or wish to have one of the devroom organizers review your presentation slides or any other content for CoC compliance, please email us and we will do our best to assist you. Questions? If you have any questions about this devroom, please send your questions to our devroom mailing list. You can also subscribe to the list to receive updates about important dates, session announcements, and to connect with other attendees. See you all at FOSDEM! [1] https://fosdem.org/submit [2] https://fosdem.org/2026/practical/conduct/

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Gueat with secure boot & PXE ?
by lejeczek 23 Oct '25

23 Oct '25

Hi guys. I'm trying to boot securely a guest - ultimately will be Windows, I hear only secure boot for win11 - via PXE but I fail to figure it out - my first foray into it. VM fails with: iPXE initialising devices... autoexec. ipxe... Not found (https:// ipxe .org/2d12618e iPXE 1.21.1+ (g5c49e) -- Open Source Network Boot Firmware —- https://ipxe.org Features: DNS HITP iSCSI TETP ULAN SRP AoE EFI Menu net@: 02:2d:7a:34:9f:90 using virtio-net on 0000:01:00.0 (Ethernet) [open] ULink:up, T:@ TKE:1 RX:@ RXE:0] ITKE: 1 x "Netuork unreachable (https://ipxe .org/28086090) "] Configuring (net 02:2d:7a:34:9F:90) ...... ok netO: 10.3.1.14/255.255.252.0 gu 10.3.1.254 netO: fe80::2d: Taff: fe34:9£90/64 Next server: 10.3.1.99 Filenane: ipxe-shinx64-ef i t£tp://10.3.1.99/ ipxe-shimx64-ef i... ok ipxe-shinx64.efi : 961448 bytes [EFI] Fetching Netboot Inage ipxe-efi Nalforned binary after Attribute Certificate Table datasize: 4194304 SunOfBytesHashed: 1044480 SecDir->Size: 1536 hashsize: 3148288 SecDir->VirtualAddress: 0x000FFO00 Failed to load image: Invalid Paraneter start_imageQ returned Invalid Paraneter, falling back to default loader Fetching Netboot Inage ipxe-efi Nalforned binary after Attribute Certificate Table datasize: 4194304 SunOfBytesHashed: 1044480 SecDir->Size: 1536 hashsize: 3148288 SecDir-VirtualAddress: 0x000FFO00 Failed to load inage: Invalid Paraneter start_imageQ returned Invalid Paraneter Guest domain is like so: ... <firmware> <feature enabled='no' name='enrolled-keys'/> <feature enabled='no' name='secure-boot'/> </firmware> <loader readonly='yes' type='pflash' format='raw'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader> <nvram template='/usr/share/edk2/ovmf/OVMF_VARS.fd' templateFormat='raw' format='raw'>/var/lib/libvirt/qemu/nvram/dzien-win-secbot_VARS.fd</nvram> ... Here boot is not secure secure - but when is secure, also fails - so I could capture pxe process which is more verbose here, as opposed to secure which pop-ups with blue screen with only a short message. I'm presuming I sign something - bootloaders |& efi vars store - wrong? I'm on centos 9 with biners up-to-today. Any/all thoughts are much appreciated. many thanks, L.

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download certificate issue
by daan.hoogland＠gmail.com 20 Oct '25

20 Oct '25

we (apache/cloudstack) are seeing issues downloading libvirt java 0.5.3 lately. It seems to be due to certificates errors on downloads.libvirt.org . I am not sure what the appropriate way to report this is, so asking here first. regards, Daan Hoogland

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Re: domain not getting IP address
by Pavel Mores 22 Sep '25

22 Sep '25

On Thu, Sep 18, 2025 at 11:29 AM Pavel Mores <pmores(a)redhat.com> wrote: > On Thu, Sep 18, 2025 at 10:25 AM Martin Kletzander <mkletzan(a)redhat.com> > wrote: > >> On Wed, Sep 17, 2025 at 04:02:12PM +0200, Pavel Mores wrote: >> >On Wed, Sep 17, 2025 at 3:05 PM Martin Kletzander <mkletzan(a)redhat.com> >> >wrote: >> > >> >> On Wed, Sep 17, 2025 at 02:14:51PM +0200, Pavel Mores via Users wrote: >> >> >Hi, >> >> > >> >> >I'm examining a domain that's connected to the 'default' network >> >> > >> >> ># virsh net-dumpxml default >> >> ><network connections='1'> >> >> > <name>default</name> >> >> > <uuid>c757baa7-2b31-4794-9dfb-0df384575602</uuid> >> >> > <forward mode='nat'> >> >> > <nat> >> >> > <port start='1024' end='65535'/> >> >> > </nat> >> >> > </forward> >> >> > <bridge name='virbr0' stp='on' delay='0'/> >> >> > <mac address='52:54:00:37:b7:92'/> >> >> > <ip address='192.168.122.1' netmask='255.255.255.0'> >> >> > <dhcp> >> >> > <range start='192.168.122.2' end='192.168.122.254'/> >> >> > </dhcp> >> >> > </ip> >> >> ></network> >> >> > >> >> >> >> This is standard. >> >> >> >> >using a device as follows: >> >> > >> >> ><interface type='network'> >> >> > <mac address='52:54:00:ed:06:2e'/> >> >> > <source network='default' >> portid='83db8ca9-baed-47f3-ba0d-1a967ee86aa5' >> >> >bridge='virbr0'/> >> >> > <target dev='vnet19'/> >> >> > <model type='virtio'/> >> >> > <alias name='net0'/> >> >> > <address type='pci' domain='0x0000' bus='0x00' slot='0x02' >> >> >function='0x0'/> >> >> ></interface> >> >> > >> >> >> >> This looks fine. >> >> >> >> >The domain is running but apparently without an IP address: >> >> > >> >> ># virsh domifaddr podvm-podsandbox-totok-8f10756a >> >> > Name MAC address Protocol Address >> >> >> >> >> >------------------------------------------------------------------------------- >> >> > >> >> >> >> This shows that libvirt does not know about any IP address. Does >> adding >> >> "--source agent", "--source arp" or "--source lease" change anything? >> >> >> > >> >'arp' and 'lease' don't but >> > >> ># virsh domifaddr --source agent podvm-podsandbox-totok-8f10756a >> >error: Failed to query for interfaces addresses >> >error: argument unsupported: QEMU guest agent is not configured >> > >> >This is surprising to me since this is a peer pods setup where the domain >> >in question is a podvm running an image which I was told does have >> >the qemu agent running. >> > >> >However the agent shouldn't be necessary for IP address acquisition I >> guess, >> >right? >> > >> >>The requisite host-side interfaces look good (to me anyway :-)): >> >> > >> >> >10: virbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc htb >> state UP >> >> >group default qlen 1000 >> >> > link/ether 52:54:00:37:b7:92 brd ff:ff:ff:ff:ff:ff >> >> > inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0 >> >> > valid_lft forever preferred_lft forever >> >> >[...] >> >> >35: vnet19: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue >> >> master >> >> >virbr0 state UNKNOWN group default qlen 1000 >> >> > link/ether fe:54:00:ed:06:2e brd ff:ff:ff:ff:ff:ff >> >> > inet6 fe80::fc54:ff:feed:62e/64 scope link proto kernel_ll >> >> > valid_lft forever preferred_lft forever >> >> > >> >> >I can share more information about the setup if necessary but I'll >> stop >> >> >here for now since I feel this must be just a simple stupid oversight >> on >> >> my >> >> >part. Please let me know if you'd like to have additional info. >> >> > >> >> >> >> When this happens to me sometimes, it's most often a firewall issue and >> >> the VM does not get any IP address or cannot communicate outside its >> >> network. >> >> >> > >> >I've seen a firewall suggested as a possible culprit, yes, however I >> don't >> >quite >> >know what it should look like. iptables appear unconfigured: >> > >> ># iptables -L -v -n >> >Chain INPUT (policy ACCEPT 0 packets, 0 bytes) >> > pkts bytes target prot opt in out source >> >destination >> > >> >Chain FORWARD (policy ACCEPT 0 packets, 0 bytes) >> > pkts bytes target prot opt in out source >> >destination >> > >> >Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes) >> > pkts bytes target prot opt in out source >> >destination >> > >> >`nft list ruleset` lists only rules that look managed by libvirt >> >itself(*). At any >> >rate the host machine has no specific hand-configured firewall that I >> know >> >of. >> > >> > >> >> What it can be here is that there are some access issues to the dnsmasq >> >> lease file. >> >> >> >> What's in your /var/lib/libvirt/dnsmasq/virbr0.status file on the host? >> >> >> > >> >It's empty. >> > >> >Thanks Martin! >> >pvl >> > >> >(*) # nft list ruleset >> >table ip libvirt_network { >> >chain forward { >> >type filter hook forward priority filter; policy accept; >> >counter packets 85854914 bytes 398726525237 jump guest_cross >> >counter packets 85854914 bytes 398726525237 jump guest_input >> >counter packets 34777368 bytes 3386943972 jump guest_output >> >} >> > >> >chain guest_output { >> >ip saddr 192.168.122.0/24 iif "virbr0" counter packets 0 bytes 0 accept >> >> This suggests there were no incoming packets from an IP address in the >> range on the bridge. >> >> >iif "virbr0" counter packets 0 bytes 0 reject >> >> And no packets from outside of that range that would fall through to >> this above rule. >> >> [...] >> >> >} >> > >> >chain guest_input { >> >> [...] >> >> >oif "virbr0" ip daddr 192.168.122.0/24 ct state established,related >> counter >> >packets 0 bytes 0 accept >> >> No packets sent to the address range on the bridge, but >> >> >oif "virbr0" counter packets 0 bytes 0 reject >> >> basically no packets sent at all. >> >> >} >> > >> >chain guest_cross { >> >iif "openshift-412" oif "openshift-412" counter packets 0 bytes 0 accept >> >iif "openshift-419" oif "openshift-419" counter packets 0 bytes 0 accept >> >iif "openshift-416" oif "openshift-416" counter packets 0 bytes 0 accept >> >iif "openshift-415" oif "openshift-415" counter packets 0 bytes 0 accept >> >iif "openshift-413" oif "openshift-413" counter packets 0 bytes 0 accept >> >iif "virbr0" oif "virbr0" counter packets 0 bytes 0 accept >> >> No intra-network communication >> >> [...] >> >> >chain guest_nat { >> >type nat hook postrouting priority srcnat; policy accept; >> >> [...] >> >> >ip saddr 192.168.122.0/24 ip daddr 224.0.0.0/24 counter packets 50 bytes >> >3676 return >> >> There were some IPv4 multicast packets, but these could've originated >> from the host. >> >> >ip saddr 192.168.122.0/24 ip daddr 255.255.255.255 counter packets 0 >> bytes >> >0 return >> >> And no broadcast packets from the address space. >> >> >meta l4proto tcp ip saddr 192.168.122.0/24 ip daddr != 192.168.122.0/24 >> >counter packets 0 bytes 0 masquerade to :1024-65535 >> >meta l4proto udp ip saddr 192.168.122.0/24 ip daddr != 192.168.122.0/24 >> >counter packets 0 bytes 0 masquerade to :1024-65535 >> >ip saddr 192.168.122.0/24 ip daddr != 192.168.122.0/24 counter packets 0 >> >bytes 0 masquerade >> >> No NAT, anything. >> >> [...] >> >ip saddr 192.168.14.0/24 ip daddr 224.0.0.0/24 counter packets 50 bytes >> >3675 return >> >> These counters on another range are the same, so I would say all the >> multicast packets on the range we are interested in are just the same, >> hence having nothing to do with the guest. >> >> >} >> >} >> >table ip6 libvirt_network { >> >chain forward { >> >type filter hook forward priority filter; policy accept; >> >counter packets 0 bytes 0 jump guest_cross >> >counter packets 0 bytes 0 jump guest_input >> >counter packets 0 bytes 0 jump guest_output >> >> And totally nothing with IPv6. >> > > As a bit of context, this is a virtlab machine whose primary purpose is to > run > kcli-based openshift clusters whose nodes are libvirt domains. Those are > the > "openshift-41[1-9]" networks and bridges. They are unrelated to the setup > I'm > looking into and most of them are actually obsolete (it's been years now > since > a 4.11 cluster last ran on the host :-)). > > My "guess" would be that the guest did not even get an IP address, maybe >> did not eve try DHCP. Are you sure the guest booted? >> > > I think it is, based on > > # virsh list > Id Name State > ------------------------------------------------- > [...] > 20 podvm-podsandbox-totok-8f10756a running > > But now that you mention it, I'm not positively sure that it tried DHCP. > The zero > traffic on the virbr0 bridge you mention above is overall explainable by > the domain > not having an address *but* if it did try DHCP those packets would show up > in the > virbr0 stats I guess? > > I did check out previously the DHCP leases on the 'default' network: > > # virsh net-dhcp-leases default > Expiry Time MAC address Protocol IP address Hostname Client ID > or DUID > > ----------------------------------------------------------------------------------- > > and there are none but that doesn't rule out any other failure in DHCP. > > The domain runs a peer pods podvm image which I don't have any control > over and > frankly am not familiar with. I assume that it does do DHCP to configure > its interfaces > but as the guest agent example shows my information about the image might > not be > always accurate. > I verified that the VM does do DHCP (there actually doesn't seem to be any other means for a podvm to get its network configured in peer pods). pvl > Is there a way to check if the domain attempts DHCP purely from the > libvirt side, just > using libvirt means? > > Thanks! > pvl >

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domain not getting IP address
by Pavel Mores 18 Sep '25

18 Sep '25

Hi, I'm examining a domain that's connected to the 'default' network # virsh net-dumpxml default <network connections='1'> <name>default</name> <uuid>c757baa7-2b31-4794-9dfb-0df384575602</uuid> <forward mode='nat'> <nat> <port start='1024' end='65535'/> </nat> </forward> <bridge name='virbr0' stp='on' delay='0'/> <mac address='52:54:00:37:b7:92'/> <ip address='192.168.122.1' netmask='255.255.255.0'> <dhcp> <range start='192.168.122.2' end='192.168.122.254'/> </dhcp> </ip> </network> using a device as follows: <interface type='network'> <mac address='52:54:00:ed:06:2e'/> <source network='default' portid='83db8ca9-baed-47f3-ba0d-1a967ee86aa5' bridge='virbr0'/> <target dev='vnet19'/> <model type='virtio'/> <alias name='net0'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/> </interface> The domain is running but apparently without an IP address: # virsh domifaddr podvm-podsandbox-totok-8f10756a Name MAC address Protocol Address ------------------------------------------------------------------------------- The requisite host-side interfaces look good (to me anyway :-)): 10: virbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc htb state UP group default qlen 1000 link/ether 52:54:00:37:b7:92 brd ff:ff:ff:ff:ff:ff inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0 valid_lft forever preferred_lft forever [...] 35: vnet19: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master virbr0 state UNKNOWN group default qlen 1000 link/ether fe:54:00:ed:06:2e brd ff:ff:ff:ff:ff:ff inet6 fe80::fc54:ff:feed:62e/64 scope link proto kernel_ll valid_lft forever preferred_lft forever I can share more information about the setup if necessary but I'll stop here for now since I feel this must be just a simple stupid oversight on my part. Please let me know if you'd like to have additional info. Thanks in advance, pvl

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