[PATCH v3 09/11] hvm/hpet: comparator can only change when master clock is enabled.

[Don Slutz <dslutz@verizon.com>]

This is based on software-developers-hpet-spec-1-0a.pdf saying:

When the main counter value matches the value in the timer's
comparator register, an interrupt can be generated.  The hardware
will then automatically increase the value in the compare register
by the last value written to that register.

When the overall enable is off (the main count is halted), none of
the compare registers should change.

The code lines:

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

are what matter here.  They will always adjust comparator to be no
more then one period away.

Using some numbers to help show the issue:

hpet_read_maincounter(h, guest_time) = 67752
period = 62500
comparator = 255252 == 67752 + 3 * 62500

comparator       : 255252
elapsed          : -125001
elapsed/period   : -2
comparator_delta : -125000
new comparator   : 130252

Signed-off-by: Don Slutz <dslutz@verizon.com>
---
v3:
  Better commit message.

 xen/arch/x86/hvm/hpet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index e24bc46..6aa6e9b 100644
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -92,7 +92,7 @@ static uint64_t hpet_get_comparator(HPETState *h, unsigned int tn,
     int64_t elapsed;
 
     comparator = h->hpet.comparator64[tn];
-    if ( timer_is_periodic(h, tn) )
+    if ( hpet_enabled(h) && timer_is_periodic(h, tn) )
     {
         /* update comparator by number of periods elapsed since last update */
         int64_t period = h->hpet.period[tn];
-- 
1.8.4