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SubjectRe: [KVM timekeeping 17/35] Implement getnsboottime kernel API
On 08/20/2010 02:02 PM, john stultz wrote:
> On Fri, 2010-08-20 at 13:37 -1000, Zachary Amsden wrote:
>
>> On 08/20/2010 08:39 AM, john stultz wrote:
>>
>>> On Thu, 2010-08-19 at 22:07 -1000, Zachary Amsden wrote:
>>>
>>>
>>>> Add a kernel call to get the number of nanoseconds since boot. This
>>>> is generally useful enough to make it a generic call.
>>>>
>>>>
>>> Few comments here.
>>>
>>>
>>>
>>>> Signed-off-by: Zachary Amsden<zamsden@redhat.com>
>>>> ---
>>>> include/linux/time.h | 1 +
>>>> kernel/time/timekeeping.c | 27 +++++++++++++++++++++++++++
>>>> 2 files changed, 28 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/include/linux/time.h b/include/linux/time.h
>>>> index ea3559f..5d04108 100644
>>>> --- a/include/linux/time.h
>>>> +++ b/include/linux/time.h
>>>> @@ -145,6 +145,7 @@ extern void getnstimeofday(struct timespec *tv);
>>>> extern void getrawmonotonic(struct timespec *ts);
>>>> extern void getboottime(struct timespec *ts);
>>>> extern void monotonic_to_bootbased(struct timespec *ts);
>>>> +extern s64 getnsboottime(void);
>>>>
>>>>
>>> So instead of converting the timespec from getboottime, why did you add
>>> a new interface? Also if not a timespec, why did you pick a s64 instead
>>> of a ktime_t?
>>>
>>>
>> The new interface was suggested several times, so I'm proposing it. I'm
>> indifferent to putting it the kernel API or making it internal to KVM.
>> KVM doesn't want to deal with conversions to / from ktime_t; this code
>> uses a lot (too much) math, and it's easy to get wrong when splitting
>> sec / nsec fields. So s64 seems a natural type for ns values. I
>> realize it's not entirely consistent with the kernel API, but s64
>> representation for ns seems to be creeping in.
>>
> I can understand wanting that, way back I was pushing for s64 ns
> representations for most time values, but the ktime_t was considered a
> reasonable compromise to avoid costly 64bit divides to split (sec,nsec)
> on 32bit arches.
>

We want time in simply parseable formats, so we always end up with sec /
msec, sec / usec, sec / nsec. This is simply a convenient
representation for humans. Programmers always end up copying this model
and it causes so many lovely bugs. How many times can you race while
reading CMOS Y/M/D/H/S?

Fortunately now that 64-bit computing is nearly pervasive, we can make
most of these problems go away.

I think gettimefromboot_ns() is a good descriptive name, but slightly
too long - it would ruin my indentation. Perhaps getrealtime_ns()?

Zach


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