I have this same issue, it’s actually only happening on a testing machine where the number
of total available vcpus is less than 16. I also know kernel-ml isn’t supported but was
wanting to do some testing with nested virt live migration which apparently is available
with the kernel 5.x line.
Does anyone know how I can manually adjust the maxcpus=16 parameter that is passed to
qemu-kvm to start VMs on this one machine?
From: Jillian Morgan <jillian.morgan(a)primordial.ca>
Sent: Wednesday, March 23, 2022 5:09 PM
Subject: [ovirt-users] 4.4.9 -> 4.4.10 Cannot start or migrate any VM (hotpluggable
cpus requested exceeds the maximum cpus supported by KVM)
After upgrading the engine from 4.4.9 to 4.4.10, and then upgrading one host, any attempt
to migrate a VM to that host or start a VM on that host results in the following error:
Number of hotpluggable cpus requested (16) exceeds the maximum cpus supported by KVM (8)
While the version of qemu is the same across hosts, (qemu-kvm-6.0.0-33.el8s.x86_64), I
traced the difference to the upgraded kernel on the new host. I have always run
elrepo's kernel-ml on these hosts to support bcache which RHEL's kernel
doesn't support. The working hosts still run kernel-ml-5.15.12. The upgraded host ran
In case anyone else runs kernel-ml, have you run into this issue?
Does anyone know why KVM's KVM_CAP_MAX_VCPUS value is lowered on the new kernel?
Does anyone know how to query the KVM capabilities from userspace without writing a
program leveraging kvm_ioctl()'s?
Related to this, it seems that ovirt and/or libvirtd always runs qmu-kvm with an -smp
argument of "maxcpus=16". This causes qemu's built-in check to fail on the
new kernel which is supporting max_vpus of 8.
Why does ovirt always request maxcpus=16?
And yes, before you say it, I know you're going to say that running kernel-ml
Jillian Morgan (she/her) 🏳️⚧️
Systems & Networking Specialist
Primordial Software Group & I.T. Consultancy