x86/vhpet: avoid 'small' time diff test on resume

It appears that even 64-bit versions of Windows 10, when not using syth-
etic timers, will use 32-bit HPET non-periodic timers. There is a test
in hpet_set_timer(), specific to 32-bit timers, that tries to disambiguate
between a comparator value that is in the past and one that is sufficiently
far in the future that it wraps. This is done by assuming that the delta
between the main counter and comparator will be 'small' [1], if the
comparator value is in the past. Unfortunately, more often than not, this
is not the case if the timer is being re-started after a migrate and so
the timer is set to fire far in the future (in excess of a minute in
several observed cases) rather then set to fire immediately. This has a
rather odd symptom where the guest console is alive enough to be able to
deal with mouse pointer re-rendering, but any keyboard activity or mouse
clicks yield no response.

This patch simply adds an extra check of 'creation_finished' into
hpet_set_timer() so that the 'small' time test is omitted when the function
is called to restart timers after migration, and thus any negative delta
causes a timer to fire immediately.

[1] The number of ticks that equate to 0.9765625 milliseconds

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index a9167581061e7e2b234fad61075648ee2b6a8068..12f13f8c3cdb9c90d49fbc6be6eb5d0040890b1e 100644 (file)
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -273,10 +273,14 @@ static void hpet_set_timer(HPETState *h, unsigned int tn,
      * Detect time values set in the past. This is hard to do for 32-bit
      * comparators as the timer does not have to be set that far in the future
      * for the counter difference to wrap a 32-bit signed integer. We fudge
-     * by looking for a 'small' time value in the past.
+     * by looking for a 'small' time value in the past. However, if we
+     * are restoring after migrate, treat any wrap as past since the value
+     * is unlikely to be 'small'.
      */
     if ( (int64_t)diff < 0 )
-        diff = (timer_is_32bit(h, tn) && (-diff > HPET_TINY_TIME_SPAN))
+        diff = (timer_is_32bit(h, tn) &&
+                vhpet_domain(h)->creation_finished &&
+                (-diff > HPET_TINY_TIME_SPAN))
             ? (uint32_t)diff : 0;
 
     destroy_periodic_time(&h->pt[tn]);