[Xen-changelog] [xen stable-4.11] x86/vhpet: avoid 'small' time diff test on resume

[patchbot@xxxxxxx]

commit 7ca58e5aa3dec342743cd0e4a8088085aefe9277
Author:     Paul Durrant <paul.durrant@xxxxxxxxxx>
AuthorDate: Fri Jul 5 10:29:12 2019 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jul 5 10:29:12 2019 +0200

    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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: b144cf45d50b603c2909fc32c6abf7359f86f1aa
    master date: 2019-05-31 11:40:52 +0200
---
 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 28377091ca..5bf54325bc 100644
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -258,10 +258,14 @@ 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]);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.11

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