[Xen-changelog] [xen master] hvm/hpet: comparator can only change when master clock is enabled.

[patchbot@xxxxxxx]

commit d03f3f93888e837e1ed2f98d9a6254ee177988eb
Author:     Don Slutz <dslutz@xxxxxxxxxxx>
AuthorDate: Fri May 2 16:18:06 2014 -0400
Commit:     Tim Deegan <tim@xxxxxxx>
CommitDate: Thu May 8 12:03:53 2014 +0100

    hvm/hpet: comparator can only change when master clock is enabled.
    
    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@xxxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Tim Deegan <tim@xxxxxxx>
---
 xen/arch/x86/hvm/hpet.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index 741fb7a..f938da9 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,
     uint64_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 */
         uint64_t period = h->hpet.period[tn];
--
generated by git-patchbot for /home/xen/git/xen.git#master

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