Don't emulate HPET with KVM

For quite some time I've experienced quite high CPU load on my KVM server with an AMD Athlon X2 BE-2400 CPU. All KVM instances used 10-15% CPU time more or less constantly. Idle load was around the 1.5-3 range (reaching over 160 with dynticks).

I've tried Virtio drivers, KSM, dynticks, time sources and lots of other tweaks, but performance have remained sluggish. Tonight I finally hit jackpot!

My server doesn't have hardware constant_tsc or HPET time source only acpi_pm:

cat /sys/devices/system/clocksource/clocksource0/available_clocksource 
acpi_pm
cat /sys/devices/system/clocksource/clocksource0/current_clocksource
acpi_pm

The simple fix is to add the -no-hpet option to KVM and now everything seems back to normal, with each guest only using 2-4% CPU time when idling (using dynticks on both host and guests).

Note 1: Apparently Windows need HPET timing.
Note 2: I had to disable dynticks for the Linux guests again, otherwise the host load is going crazy during the nightly backup job with loads of 30+.
Note 3: Actually setting the kernel parameter clocksource=jiffies is enough to get back old performance without having to disable HPET emulation.