Re: [patch 3/3] PTP: add kvm PTP driver

Marcelo Tosatti
2017-01-16T19:37:54Z

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?