[Xen-devel] [PATCH v3] x86/hvm/hpet: avoid 'small' time diff test on resume

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>
Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>

I notice that we seemingly don't handle main counter wrap in the HPET code.
The spec. says that timers should fire at the point the counter wraps at the
timer's width. I think the need for the 'small' time test would go away if
this was implemented, but that's for another day.
---
 xen/arch/x86/hvm/hpet.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index a916758106..12f13f8c3c 100644
--- 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]);
-- 
2.20.1.2.gb21ebb671


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