x86/HPET: don't needlessly set up channels for broadcast

When there are more FSB delivery capable HPET channels than CPU cores
(or threads), we can simply use a dedicated channel per CPU. This
avoids wasting the resources to handle the excess channels (including
the pointless triggering of the respective interrupt on each
wraparound) as well as the ping-pong of the interrupts' affinities
(when getting assigned to different CPUs).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hpet.c
+++ b/xen/arch/x86/hpet.c
@@ -369,7 +369,7 @@ static void __init hpet_fsb_cap_lookup(v
     if ( !hpet_events )
         return;
 
-    for ( i = 0; i < num_chs; i++ )
+    for ( i = 0; i < num_chs && num_hpets_used < nr_cpu_ids; i++ )
     {
         struct hpet_event_channel *ch = &hpet_events[num_hpets_used];
         u32 cfg = hpet_read32(HPET_Tn_CFG(i));
@@ -408,6 +408,9 @@ static struct hpet_event_channel *hpet_g
     if ( num_hpets_used == 0 )
         return hpet_events;
 
+    if ( num_hpets_used >= nr_cpu_ids )
+        return &hpet_events[cpu];
+
     do {
         next = next_channel;
         if ( (i = next + 1) == num_hpets_used )