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

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Message-ID: <20170116193655.GA7649@amt.cnet>
Date:   Mon, 16 Jan 2017 17:36:55 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Radim Krcmar <rkrcmar@...hat.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 3/3] PTP: add kvm PTP driver

On Mon, Jan 16, 2017 at 07:01:48PM +0100, Radim Krcmar wrote:
> > Sorry the clock difference is 10ns now. So the guest clock is off by _10 ns_ 
> > of the host clock.
> 
> That is pretty good.

Yes.

> > You are suggesting to use getcrosststamp instead, to drop the (rdtsc() -
> > guest_tsc) part ?
> 
> Yes, it results in simpler code, doesn't create dependency on the
> dreaded kvmclock, and is the best we can currently do wrt. precision.

Sorry, unless i am misunderstanding how this works, it'll get the guest clock
2us behind, which is something not wanted.

Miroslav, if ->gettime64 returns the host realtime at 2us in the past, 
this means Chrony will sync the guest clock to

host realtime - 2us

Is that correct?