Hyper-V: Kernel panic on AMD Zen2 when dynamically loading a kernel module with more than 1 vCPU configured (e.g. hv_hid or mounting nullfs or unionfs)

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• I have read the contributing guide lines at https://github.com/opnsense/src/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/src/issues?q=is%3Aissue

Describe the bug

When installing or booting OPNsense 26.7 RC1 on Hyper-V it panics when unionfs or nullfs is mounted.

This happens when 2 or more vCPUs are configured. With 1 vCPU these issues do not happen.

This can happen during boot of the live installer image (unionfs) or when the installation succeeds e.g. if Unbound starts (nullfs).

The hv_hidkernel module can also cause kernel crashed during boot with 2vCPUs.

To Reproduce

Steps to reproduce the behavior:

• Configure a Hyper-V VM with 2 vCPUs and boot and installed OPNsense 26.7 RC1 system
• Ensure Unbound is disabled during boot
• execute this command mount -r -t nullfs /usr/local/lib/python3.13 /mnt (just an example, can be anything really)
• System panics when mounting nullfs

Screenshots

There is an upstream report about this CPU behavior and Hyper-V, but no connection to mounting a temporary filesystem:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=291577

Start by reproducing the panic on an AMD Zen2 Hyper-V VM running OPNsense 26.7 RC1 with 2+ vCPUs, using the nullfs mount command or loading unionfs or hv_hid. Read the linked FreeBSD report 291577 alongside the panic screenshot and compare behavior with one vCPU. Done means the affected module-loading or filesystem scenarios no longer panic with multiple vCPUs.