Comment 43 for bug 795717

In comment #2 https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/795717/comments/2
gdahlman's observation:
{{ As an update I can boot when the guest is restricted to a single cpu ... }}

I think means that on the VM host if /usr/bin/kvm is forced to run on only one CPU via setting its processor affinity (via /usr/bin/taskset from the util-linux package) this issue does not occur. I can and will test that too soon.

If it's true it explains a bit of why Ubuntu VM guests only sometime hang. The /usr/bin/kvm process running them might only sometimes switch CPUs while the VM guest is rebooting while /usr/bin/kvm running 32-bit RHEL 5 with 2 CPUs might always switch CPUs while the VM guest is rebooting.

Also of note, 64-bit RHEL 5 does not use kvm-clock as a clocksource. For example:

64-bit RHEL 5
$ sudo cat /sys/devices/system/clocksource/clocksource0/available_clocksource
jiffies

32-bit RHEL 5
$ sudo cat /sys/devices/system/clocksource/clocksource0/available_clocksource
acpi_pm jiffies hpet tsc pit kvm-clock

Both x86 and x86_64 RHEL 6 use kvm-clock but also linux kernel 2.6.32

I think by not using kvm-clock 64-bit RHEL 5 is not 100% vulnerable the way 32-bit RHEL 5 is.