[PATCH v2 09/10] hvm/hpet: Correctly limit period to a maximum.

[Don Slutz <dslutz@verizon.com>]

In the code section after the comment:

    /*
     * Clamp period to reasonable min/max values:
     *  - minimum is 100us, same as timers controlled by vpt.c
     *  - maximum is to prevent overflow in time_after() calculations
     */

The current maximum limit actually allows "bad" values like 0 and 1.
This is because it uses a mask not a maximum.

Signed-off-by: Don Slutz <dslutz@verizon.com>
---
 xen/arch/x86/hvm/hpet.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index 28f5224..3226a61 100644
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -419,6 +419,9 @@ static int hpet_write(
             }
             else
             {
+                uint64_t max_period =
+                    (timer_is_32bit(h, tn) ? ~0u : ~0ull) >> 1;
+
                 /*
                  * Clamp period to reasonable min/max values:
                  *  - minimum is 100us, same as timers controlled by vpt.c
@@ -427,7 +430,8 @@ static int hpet_write(
                  */
                 if ( hpet_tick_to_ns(h, new_val) < MICROSECS(100) )
                     new_val = (MICROSECS(100) << 10) / h->hpet_to_ns_scale;
-                new_val &= (timer_is_32bit(h, tn) ? ~0u : ~0ull) >> 1;
+                if ( new_val > max_period )
+                    new_val = max_period;
                 h->hpet.period[tn] = new_val;
             }
         }
-- 
1.8.4