AMD server do not support nested virtualization

[bryan]

From Linux kernel v4.19 onwards, the nested KVM parameter is enabled by default for Intel and AMD. (Though your Linux distribution might override this default, here is the official documentation of this: https://www.kernel.org/doc/html/v5.7/virt/kvm/running-nested-guests.html

We are using OpenStack Zed on CentOS 9 and the VM is running on AMD compute nodes, and the kernel version is: 5.14.0-386.el9.x86_64.

When we created an instance on AMD server and set the "cpu_mode" to "none", we found that the "svm" feature is passed to the instance XML on libvirt, but when we execu "lscpu" inside the VM, we can not see the "svm" feature, so we could not create a L2 instance inside the VM.

However, when we set the "cpu_mode" to "host-passthrough" and hard reboot the VM, the "svm" is set correctly within the VM.

For intel servers, we can create nested instances by default, and the "cpu_mode" is also set to "none", and everything works well.

We guess it might because of some CPU feature dependencies which cause this issue. Can you help us to take a look? Thanks