kvm hangs when rebooting a guest VM

[Bernhard Schmidt <berni@birkenwald.de>]

Hello,

I have a small but annoying issue.

Host:
* AMD Athlon64
* Debian Lenny, Kernel 2.6.26-1-amd64
* kvm-72 (Debian package) or kvm-76 (self-built)

Guest:
* monolithic vanilla 2.6.26, KVM_GUEST and KVM_CLOCK, VIRTIO for both
  block device and NIC
* various Debian and Ubuntu versions

I don't use any libvirt or similar things, I start the VMs by directly
calling kvm in a script. I directly specify the kernel and it's options
with the -kernel and -append parameters.

Problem:
Shutting down a VM from inside it (shutdown -h now) works just fine.

Will now halt.
[171368.812319] ACPI: Preparing to enter system sleep state S5
[171368.816964] Power down.
[171368.820177] acpi_power_off called

then the kvm process exits. However, restarting the VM (reboot) does not
work ...

Will now restart.
[   37.088270] Restarting system.
[   37.089775] machine restart

this is where the KVM process hangs in a loop and uses 100% CPU on the
host. I can still kill it (Ctrl-A x) or change to it's console, but
system_restart doesn't seem to work there either.

iggy suggested to use an extboot.bin from an earlier kvm version (I used
kvm-60), didn't change.

Any idea what could be wrong here?

Regards,
Bernhard