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

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Message-ID: <20170116154814.GA31452@potion>
Date:   Mon, 16 Jan 2017 16:48:15 +0100
From:   Radim Krcmar <rkrcmar@...hat.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 3/3] PTP: add kvm PTP driver

2017-01-14 16:26+0100, Richard Cochran:
> On Fri, Jan 13, 2017 at 04:56:58PM +0100, Radim Krcmar wrote:
>> > +static int __init ptp_kvm_init(void)
>> > +{
>> > +	if (!kvm_para_available())
>> > +		return -ENODEV;
>> > +
>> > +	kvm_ptp_clock.caps = ptp_kvm_caps;
>> > +
>> > +	kvm_ptp_clock.ptp_clock = ptp_clock_register(&kvm_ptp_clock.caps, NULL);
>> 
>> It is a shame that the infrastructure uses polling when the guest could
>> be notified on every host real time change, but this should be good
>> enough.
> 
> This comment makes no sense at all.  What do you mean by "host real
> time change"?

Real time in the sense of wall clock, as perceived by the host, and the
change of that time.

Unlike other PTP drivers, host (source) and guest (destination) share
the same hardware clock, so they cannot shift or drift unless one of
them changes its "TSC to real time" conversion (the host is most likely
using NTP/PTP to keep its own real time).

I meant that the host could notify the guest when a change happens,
which would be more efficient.