Re: [Xen-devel] [PATCH v3 10/11] hvm/hpet: Prevent master clock equal to comparator while enabled

["Slutz, Donald Christopher" <dslutz@xxxxxxxxxxx>]

On 04/25/14 08:25, Jan Beulich wrote:
>>>> On 17.04.14 at 19:43,<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.
> So this is basically answering the question I raised on an earlier
> patch, and I think it implies that this earlier patch is then pointless
> (or should likely be folded with this one).

Yes, the -1 is wrong.

After more thinking on it, the last patch (#11) does make the previous 
patch (#8) not needed.

Looks like I will be dropping patch #8 "hvm/hpet: Use signed divide in 
hpet_get_comparator."
as not needed.

    -Don Slutz



> Jan
>

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