please revert c/s 17686

["Jan Beulich" <jbeulich@novell.com>]

Switching HPET into legacy mode is not acceptable, as this doesn't only
cut off the PIT channel 0 interrupt, but also the RTC one. The former is
acceptable as no domain will ever gain control over it, but the latter isn't
as Dom0 may validly make use of it. I'm observing a failure of setting the
system clock correctly due to this issue (hwclock checks whether the RTC
update interrupt is occurring as expected), and I suppose other uses of
/dev/rtc would also suffer.

It is my understanding that using the HPET to overcome the APIC timer
stop issue is therefore impossible at present - you cannot use legacy
mode, and you also cannot use the individual routing mode as whatever
IRQ is chosen may turn out being in use by one or more other devices
(and hence would require sharing the IRQ between Xen and one or more
guests).

Thanks, Jan