[Bug 263072] System freezes during reboot when vmm module is loaded inside a QEMU/KVM virtual machine

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263072

            Bug ID: 263072
           Summary: System freezes during reboot when vmm module is loaded
                    inside a QEMU/KVM virtual machine
           Product: Base System
           Version: 13.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: bhyve
          Assignee: virtualization@FreeBSD.org
          Reporter: dasebek@gmail.com

FreeBSD system running inside a QEMU/KVM virtual machine freezes during a
reboot when the vmm kernel module is loaded.

- How reproducible:

Always

- How to reproduce:

1. Install FreeBSD 13.0-RELEASE or 13.1-RC1
2. run the following commands as root:
# kldload vmm
# reboot

- Expected behavior:

The system reboots.

- Observed behavior:

The reboot proceeds as usual up until the point when the "Rebooting..." message
is displayed and the machine should restart. But instead of restarting, the
screen goes blank, but the CPU usage remains high, and the virtual machine
hangs until I do a hard power-off of the virtual machine.

- Affected versions:

Guest OS:
The problem occurred on all FreeBSD versions that I tested. Namely,
FreeBSD 13.1 built from releng/13.1 on April 4, 2022
FreeBSD 13.0-RELEASE
FreeBSD 13.0-STABLE built in October 2021

Hypervisor:
I used QEMU version 6.2.0 (qemu-6.2.0-5.fc36) together with virt-manager
version 4.0.0. It doesn't matter whether I choose the Q35 or i440FX chipset or
BIOS or UEFI boot in the VM settings.
The problem is also present when running FreeBSD in a DigitalOcean droplet.

I don't know how loading the vmm module affects the system reboot. It could
also be an issue with the QEMU/KVM hypervisor itself. But without the vmm
module loaded, the machine reboots correctly every time.

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