[DTrace-devel] [PATCH v2] Add support for built-in variable walltimestamp

[Kris Van Hees]

On Tue, Jun 15, 2021 at 01:25:27PM -0400, Eugene Loh wrote:
> On 6/15/21 11:08 AM, Kris Van Hees wrote:
> 
> > On Mon, Jun 14, 2021 at 05:52:01PM -0400, eugene.loh at oracle.com wrote:
> >> From: Eugene Loh <eugene.loh at oracle.com>
> >>
> >> In the pre-compiled get_bvar() function, use bpf_ktime_get_ns() to
> >> get the time (in nsec) since boot.  This value is adjusted by adding
> >> the POSIX time when the system was booted.
> >>
> >> diff --git a/bpf/get_bvar.c b/bpf/get_bvar.c
> >> +extern uint64_t BOOTTIMEHI;
> >> +extern uint64_t BOOTTIMELO;
> >> @@ -167,6 +169,10 @@ noinline uint64_t dt_get_bvar(dt_dctx_t *dctx, uint32_t id)
> >> +	case DIF_VAR_WALLTIMESTAMP:
> >> +		return bpf_ktime_get_ns()
> >> +		    + (((uint64_t) &BOOTTIMEHI) << 32)
> >> +		    + ((uint64_t) &BOOTTIMELO);
> > Why do you use two variables when each is 64-bit bits wide and the adjustment
> > itself is a 64-bit value.  So, a single variable ought to be enough.
> Yup.  That's what I thought.  Nevertheless, only the lowest 32 bits 
> survive.  I guess I can investigate more, but the single variable did 
> not work for me.  To be picky, the width of the variable is 
> unimportant;  we want the address.  But yeah, I agree with you, but it 
> just did not work.

Ah, but that means we have a bug...  Which we should fix instead of working
around it :)  Anyway, patch on its way soon.