Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

[David Woodhouse <dwmw2@infradead.org>]

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On Thu, 2024-06-20 at 17:19 +0100, David Woodhouse wrote:
> 
> > 
> > > +
> > > +       /* Counter frequency, and error margin. Units of (second >> 64) */
> > > +       uint64_t counter_period_frac_sec;
> > 
> > AFAIU this might limit the precision in case of high counter frequencies.
> > Could the unit be aligned to the expected frequency band of counters?
> 
> This field indicates the period of a single tick, in units of 1>>64 of
> a second. That's about 5.4e-20 seconds, or 54 zeptoseconds? 
> 
> Can you walk me through a calculation where you believe that level of
> precision is insufficient?
> 
> I guess the precision matters if the structure isn't updated for a long
> period of time, and the delta between the current counter and the
> snapshot is high? That's a *lot* of 54 zeptosecondses? But you really
> would need a *lot* of them before you care? And if nobody's been
> calibrating your counter for that long, surely you have bigger worries?
> 
> Am I missing something there?

Hm, that was a bit rushed at the end of the day; let's take a better look...

Let's take a hypothetical example of a 100GHz counter. That's two
orders of magnitude more than today's Arm arch counter.

The period of such a counter would be 10 picoseconds. 

(Let's ignore the question of how far light actually travels in that
time and how *realistic* that example is, for the moment.)

It turns out that at that rate, there *are* a lot of 54 zeptosecondses
of precision loss in the day. It could be half a millisecond a day, or
20µs an hour.

That particular example of 10 picoseconds is 184467440.7370955
(seconds>>64) which could be truncated to 184467440 — losing about 4PPB
(a third of a millisecond a day; 14µs an hour).

So yeah, I suppose a 'shift' field could make sense. It's easy enough
to consume on the guest side as it doesn't really perturb the 128-bit
multiplication very much; especially if we don't let it be negative.

And implementations *can* just set it to zero. It hurts nobody.

Or were you thinking of just using a fixed shift like (seconds>>80)
instead?

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