Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross timestamps infrastructure

From: Richard Cochran
Date: Tue Jan 24 2017 - 00:43:14 EST


On Mon, Jan 23, 2017 at 08:44:53PM +0100, Paolo Bonzini wrote:
> If you just implement getclock64 the PTP_SYS_OFFSET output:
>
> device clock | |sample2| |sample4| |sample6| ...
> -------------------------------------------------------------
> realtime clock |sample1| |sample3| |sample5|
>
> has a very large distance between samples on the same line (about 1 us),
> and I think it is too noisy for userspace to make sense of the output.

One microsecond is not too bad at all. The PCIe devices have
intervals of 5-6 usec, and the result is quite usable, certainly
better than generic NTP.

> Marcelo's patch then produces fake realtime clock samples that, however,
> let chrony derive the cross timestamps that KVM produced in the first
> place. The outcome is really great accuracy compared to previous
> versions of the patch, often just +/- 2 or 3 nanoseconds.

Why can't chrony learn about PTP_SYS_OFFSET_PRECISE?

Thanks,
Richard