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

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Message-ID: <20170124054302.GB1906@localhost.localdomain>
Date:   Tue, 24 Jan 2017 06:43:03 +0100
From:   Richard Cochran <richardcochran@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Radim Krcmar <rkrcmar@...hat.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross
 timestamps infrastructure

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