[PATCH v3 08/11] hvm/hpet: Use signed divide in hpet_get_comparator.

[Don Slutz <dslutz@verizon.com>]

This is done so that when comparator is less then or equal to one
period it does not change.

The code lines:

    elapsed = hpet_read_maincounter(h, guest_time) +
        period - 1 - comparator;
    comparator += (elapsed / period) * period;

are what matter here.  For almost all cases where
"hpet_read_maincounter() + period - 1" is greater then
"comparator", a signed divide will produce the same answer as an
unsigned divide.  One of the problem areas is when
"hpet_read_maincounter() + period - 1" needs more then 64 bits to
represent it.  It includes cases where hpet_read_maincounter() has
wrapped (I.E. needs more then 64 bits to correctly represent it),
but "comparator" has not wrapped.  However "elapsed" is correctly
greater then zero as long as the mathematical result only takes 63
bits to represent it.

It is safe to declare period as a signed integer because the code
already limits it to 63 bits.

Using some numbers to help show the issue:

hpet_read_maincounter(h, guest_time) = 67752
period = 62500
comparator = 130252 == 67752 + 62500

what               unsigned               signed

comparator       : 130252                  130252
elapsed          : 18446744073709551615    -1
elapsed/period   : 295147905179352         0
comparator_delta : 18446744073709500000    0
new comparator   : 78636                   130252

Signed-off-by: Don Slutz <dslutz@verizon.com>
---
 xen/arch/x86/hvm/hpet.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index 7964df2..e24bc46 100644
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -89,13 +89,13 @@ static uint64_t hpet_get_comparator(HPETState *h, unsigned int tn,
                                     uint64_t guest_time)
 {
     uint64_t comparator;
-    uint64_t elapsed;
+    int64_t elapsed;
 
     comparator = h->hpet.comparator64[tn];
     if ( timer_is_periodic(h, tn) )
     {
         /* update comparator by number of periods elapsed since last update */
-        uint64_t period = h->hpet.period[tn];
+        int64_t period = h->hpet.period[tn];
         if (period)
         {
             elapsed = hpet_read_maincounter(h, guest_time) +
-- 
1.8.4