hvm/hpet: Prevent master clock equal to comparator while enabled

Based on the software-developers-hpet-spec-1-0a.pdf, the comparator
for a periodic timer will change to the new value when it matches
the master clock.  The current code here uses a very standard
rounding formula of "((x + y - 1) / y) * y".  This is wrong because
in this case you need to go to the next comparator value when "x"
equals "y". Not when "x + 1" equals "y".  In this case "y" is the
period and "x" is the master clock.

The code lines:

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

are what matter here.

Using some numbers to help show the issue:

hpet_read_maincounter(h, guest_time) = 130252
period = 62500

comparator       : 130252
elapsed          : 62499
elapsed/period   : 0
comparator_delta : 0
new comparator   : 130252

Signed-off-by: Don Slutz <dslutz@verizon.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index f938da977a7d12426eaffb471f27cfc38583bdb2..2a99fed8c6ed6a11776ddd9bdbc506407797d127 100644 (file)
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -99,7 +99,7 @@ static uint64_t hpet_get_comparator(HPETState *h, unsigned int tn,
         if (period)
         {
             elapsed = hpet_read_maincounter(h, guest_time) +
-                period - 1 - comparator;
+                period - comparator;
             comparator += (elapsed / period) * period;
             h->hpet.comparator64[tn] = comparator;
         }