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 kernel-ml-5.17.0.

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
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 isn't supported.

Jillian Morgan (she/her) 🏳️‍⚧️
Systems & Networking Specialist
Primordial Software Group & I.T. Consultancy
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