Re: [Xen-devel] [PATCH v3 5/5] xen/console: Traditional console timestamps including milliseconds

[Don Slutz <dslutz@xxxxxxxxxxx>]

On 03/06/14 19:41, Andrew Cooper wrote:
> On 06/03/2014 23:54, Don Slutz wrote:
> > On 03/06/14 14:28, Andrew Cooper wrote:
> > >    }
> > >    -struct tm wallclock_time(void)
> > > +struct tm wallclock_time(uint64_t *ns)
> > >    {
> > Adding:
> >
> >     if ( ns )
> >        *ns = 0;
> >
> > Makes sense here.
> >
> > >        return (struct tm) { 0 };
> > >    }
> > > diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> > > index 4f4de22..1156ccc 100644
> > > --- a/xen/arch/x86/time.c
> > > +++ b/xen/arch/x86/time.c
> > > @@ -1646,15 +1646,19 @@ int dom0_pit_access(struct ioreq *ioreq)
> > >        return 0;
> > >    }
> > >    -struct tm wallclock_time(void)
> > > +struct tm wallclock_time(uint64_t *ns)
> > >    {
> > > -    uint64_t seconds;
> > > +    uint64_t seconds, nsec;
> > >          if ( !wc_sec )
> > And here.
> >
> > >            return (struct tm) { 0 };
> > >          seconds = NOW() + SECONDS(wc_sec) + wc_nsec;
> > > -    do_div(seconds, 1000000000);
> > > +    nsec = do_div(seconds, 1000000000);
> > > +
> > > +    if ( *ns )
> > This should be just
> >
> >     if ( ns )
> Oops - so it should.
>
> As for the other changes, I am a bit ambivalent.  printk_start_of_line()
> is the sole caller of wallclock_time(), which means that tm.tm_day being
> 0 means *ns will never get looked at.
This is not 100% true.  Last I knew (struct tm) { 0 } is

struct tm {
    int     tm_sec;         /* seconds */ = 0
    int     tm_min;         /* minutes */ <undef>
    int     tm_hour;        /* hours */ <undef>
    int     tm_mday;        /* day of the month */ <undef>
    int     tm_mon;         /* month */ <undef>
    int     tm_year;        /* year */ <undef>
    int     tm_wday;        /* day of the week */ <undef>
    int     tm_yday;        /* day in the year */ <undef>
    int     tm_isdst;       /* daylight saving time */ <undef>
};

But not having looked at the code, and the compilers could have changed 
what this means (i.e. if not provided they are zero in which case this 
should be {}.  { 0, 0, 0, 0 } or { .tm_mday = 0 } is the way I know of 
to say "tm_mday" is zero).
So the test for tm.tm_mday == 0 may just be working because the 
undefined value is zero...
> While it is not exactly a hot codepath, unconditionally clearing it
> seems silly, especially as it is not exactly the most likely candidate
> to get a new caller in the near future.
There is no clear right answer here.  So I have no issue with not doing 
the "extra" work.
   -Don Slutz

> ~Andrew

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