linux-kernel - Re: [patch 3/3] PTP: add kvm PTP driver

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Date:   Wed, 18 Jan 2017 16:54:52 +0100
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [patch 3/3] PTP: add kvm PTP driver

On Wed, Jan 18, 2017 at 12:24:09PM -0200, Marcelo Tosatti wrote:
> On Wed, Jan 18, 2017 at 01:46:58PM +0100, Paolo Bonzini wrote:
> > I'd rather use PTP_SYS_OFFSET_PRECISE instead, but unfortunately chrony
> > does not support it---but I would still prefer you to support
> > PTP_SYS_OFFSET_PRECISE as well.
> 
> Sure, I'll check if it makes sense to implement PTP_SYS_OFFSET_PRECISE for 
> KVM case.

But is it really so precise that the application can safely assume
there are no errors due to asymmetric delays, etc? I think
PTP_SYS_OFFSET_PRECISE should be supported only if the accuracy of
the offset measured between the HW and system clock is not worse than
the precision of the system clock (typically few tens of nanoseconds).

It would be good to verify the accuracy of the offset when the host
and guest clocks are synchronised to each other over PTP using two
NICs with HW timestamping.

-- 
Miroslav Lichvar