Don't enable a HPET timer if HPET is disabled

A HPET timer can be started when HPET is not yet
enabled. This will not generate an interrupt
to the guest, but causes problems when HPET is later
enabled.

A timer that is created and expires at least once before
HPET is enabled will have an initialized comparator based
on a hpet_offset of 0 (uninitialized). When HPET is
enabled, hpet_set_timer() is called a second time, which
modifies the timer expiry to a time based on the
difference between current ticks (measured with the
newly initialized hpet_offset) and the timer's
comparator (which was generated before hpet_offset was
initialized). This results in a long period of no HPET
timer ticks.

When this occurs with a CentOS 5.x guest, the guest
may not receive timer interrupts during its narrow
timer check window and panic on boot.
Signed-off-by: NMatt Lupfer <mlupfer@ddn.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/timer/hpet.c

@@ -506,7 +506,8 @@ static void hpet_ram_write(void *opaque, hwaddr addr,
                 timer->cmp = (uint32_t)timer->cmp;
                 timer->period = (uint32_t)timer->period;
             }
-            if (activating_bit(old_val, new_val, HPET_TN_ENABLE)) {
+            if (activating_bit(old_val, new_val, HPET_TN_ENABLE) &&
+                hpet_enabled(s)) {
                 hpet_set_timer(timer);
             } else if (deactivating_bit(old_val, new_val, HPET_TN_ENABLE)) {
                 hpet_del_timer(timer);