[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH v3 10/11] hvm/hpet: Prevent master clock equal to comparator while enabled
Based on the software-developers-hpet-spec-1-0a.pdf, the comparator for a periodic timer will change to the new value when it matches the master clock. The current code here uses a very standard rounding formula of "((x + y - 1) / y) * y". This is wrong because in this case you need to go to the next comparator value when "x" equals "y". Not when "x + 1" equals "y". In this case "y" is the period and "x" is the master clock. The code lines: elapsed = hpet_read_maincounter(h, guest_time) + period - 1 - comparator; comparator += (elapsed / period) * period; are what matter here. Using some numbers to help show the issue: hpet_read_maincounter(h, guest_time) = 130252 period = 62500 comparator : 130252 elapsed : 62499 elapsed/period : 0 comparator_delta : 0 new comparator : 130252 Signed-off-by: Don Slutz <dslutz@xxxxxxxxxxx> --- 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 6aa6e9b..50047f5 100644 --- a/xen/arch/x86/hvm/hpet.c +++ b/xen/arch/x86/hvm/hpet.c @@ -99,7 +99,7 @@ static uint64_t hpet_get_comparator(HPETState *h, unsigned int tn, if (period) { elapsed = hpet_read_maincounter(h, guest_time) + - period - 1 - comparator; + period - comparator; comparator += (elapsed / period) * period; h->hpet.comparator64[tn] = comparator; } -- 1.8.4 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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