hvm: hpet: Fix overflow when converting to nanoseconds.

Currently in hpet_tick_to_ns, the approach is multiplying first, which
easily causes overflow when tick is quite large. The patch cannot
handle arbitratry large ticks duo to the precision requirement and
64bit's value range. But by optimize the equation, a larger ticks than
current code can be supported. Also an overflow check is added before
the calculation.

From: Haitao Shan <haitao.shan@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index 603ffe8915..14f525893f 100644
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -71,8 +71,9 @@
 #define HPET_TN_INT_ROUTE_CAP_MASK (0xffffffffULL \
                     << HPET_TN_INT_ROUTE_CAP_SHIFT)
 
-#define hpet_tick_to_ns(h, tick) ((s_time_t)(tick)* \
-                                  (S_TO_NS*TSC_PER_HPET_TICK)/h->tsc_freq)
+#define hpet_tick_to_ns(h, tick)                        \
+    ((s_time_t)((((tick) > (h)->hpet_to_ns_limit) ?     \
+        ~0ULL : (tick) * (h)->hpet_to_ns_scale) >> 10))
 
 #define timer_config(h, n)       (h->hpet.timers[n].config)
 #define timer_is_periodic(h, n)  (timer_config(h, n) & HPET_TN_PERIODIC)
@@ -537,6 +538,9 @@ void hpet_init(struct vcpu *v)
     h->vcpu = v;
     h->tsc_freq = ticks_per_sec(v);
 
+    h->hpet_to_ns_scale = ((S_TO_NS * TSC_PER_HPET_TICK) << 10) / h->tsc_freq;
+    h->hpet_to_ns_limit = (~0ULL >> 1) / h->hpet_to_ns_scale;
+
     /* 64-bit main counter; 3 timers supported; LegacyReplacementRoute. */
     h->hpet.capability = 0x8086A201ULL;