x86/hpet: clear unwanted bits

Leaving certain bits set when being started from an environment where
the HPET was already in use can affect functionality. Clear those bits
to be on the safe side.

We should also consider ignoring the HPET altogether if any reserved
bits are found to be set.

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

--- a/xen/arch/x86/hpet.c
+++ b/xen/arch/x86/hpet.c
@@ -537,7 +537,7 @@ void __init hpet_broadcast_init(void)
     {
         /* set HPET Tn as oneshot */
         cfg = hpet_read32(HPET_Tn_CFG(hpet_events[i].idx));
-        cfg &= ~HPET_TN_PERIODIC;
+        cfg &= ~(HPET_TN_LEVEL | HPET_TN_PERIODIC | HPET_TN_FSB);
         cfg |= HPET_TN_ENABLE | HPET_TN_32BIT;
         hpet_write32(cfg, HPET_Tn_CFG(hpet_events[i].idx));
 
@@ -594,7 +594,7 @@ void hpet_broadcast_resume(void)
 
         /* set HPET Tn as oneshot */
         cfg = hpet_read32(HPET_Tn_CFG(hpet_events[i].idx));
-        cfg &= ~HPET_TN_PERIODIC;
+        cfg &= ~(HPET_TN_LEVEL | HPET_TN_PERIODIC | HPET_TN_FSB);
         cfg |= HPET_TN_ENABLE | HPET_TN_32BIT;
         hpet_write32(cfg, HPET_Tn_CFG(hpet_events[i].idx));
 
--- a/xen/include/asm-x86/hpet.h
+++ b/xen/include/asm-x86/hpet.h
@@ -32,9 +32,11 @@
 #define	HPET_LEGACY_8254	2
 #define	HPET_LEGACY_RTC		8
 
+#define HPET_TN_LEVEL		0x002
 #define HPET_TN_ENABLE		0x004
 #define HPET_TN_PERIODIC	0x008
 #define HPET_TN_PERIODIC_CAP	0x010
+#define HPET_TN_64BIT_CAP	0x020
 #define HPET_TN_SETVAL		0x040
 #define HPET_TN_32BIT		0x100
 #define HPET_TN_ROUTE		0x3e00