Re: [KVM timekeeping 17/35] Implement getnsboottime kernel API

[Zachary Amsden]

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@xxxxxxxxxx>
> > > > ---
> > > >    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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