Hi everyone In KVM it is possible to select the alarm timer being used (dynticks, hpet, rtc, unix).

Is someone already working on the support of this for libvirt?

If not I'll see that I can come up with patches for libvirt and virt-inst within the next month (if time permits).

On Sun, Aug 24, 2008 at 08:20:05PM +0100, Daniel P. Berrange wrote: > On Sun, Aug 24, 2008 at 08:40:46PM +0200, Tiziano M?ller wrote: > > Hi everyone > > > > In KVM it is possible to select the alarm timer being used (dynticks, > > hpet, rtc, unix). > > What exactly do these options do ? Is there anything describing the > pros/cons of the different options & is there a way to determine > what the best option is for a particular VM ? I've had a long look at the source code in KVM-73 associated with these timers and have some comments: - QEMU tests the available timer sources in order and chooses the first one which works (allegedly the "best" one). The order, on Linux, is: Timer Implementation method --------------------------------------------------------------- (1) dynticks POSIX timer_create (2) hpet Linux-specific hardware (/dev/hpet) - see below (3) rtc old /dev/rtc, millisecond accuracy (4) "unix" POSIX setitimer - Dynticks is at the top of the list, and has an extra 'rearm' operation (the others only support starting and stopping timers). But I don't understand how this rearm operation is an improvement. - HPET is second in the list and most accurate. It's backed by hardware and has sub-microsecond accuracy. - It's not clear if dynticks (== timer_create) uses HPET internally in the kernel? - I can't see how any of this would affect guests -- except that choosing a more accurate timer would be in some way 'better' because the guest would then get more accurate emulation events. > Basically I'm wondering where/if this should be exposed in the XML > format, or whether libvirt could/should just internally pick the > best one ? I can't see how libvirt can do any better than qemu/kvm is doing already. So unless someone comes along with an argument that using (eg) hpet is better for Windows guests or something, then I suggest that libvirt doesn't need to do anything.