Re: [Xen-devel] [PATCH v4 09/10] hvm/hpet: Prevent master clock equal to comparator while enabled

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.05.14 at 22:18, <dslutz@xxxxxxxxxxx> wrote:
> 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>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

> ---
>  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 f938da9..2a99fed 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




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