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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Jan 18, 2017 at 03:02:23PM +0100, Paolo Bonzini wrote:
> 
> 
> On 18/01/2017 14:36, Miroslav Lichvar wrote:
> > On Wed, Jan 18, 2017 at 01:46:58PM +0100, Paolo Bonzini wrote:
> >> On 18/01/2017 13:24, Marcelo Tosatti wrote:
> >>>> Testcase: run a guest and a loop sending SIGUSR1 to vcpu0 (emulating
> >>>> intense interrupts). Follows results:
> > 
> >>>> Do you still want to drop it in favour of simplicity?
> > 
> >> It's just that it's not obvious why you get better results with biased
> >> host timestamps.  What makes the biased host timestamp more precise?
> >>
> >> 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.
> > 
> > Interesting. I wasn't aware that there is a new ioctl for measuring
> > the HW-sys offset. Adding support to chrony shouldn't be difficult.
> > 
> > If I understand it correctly, PTP_SYS_OFFSET can be emulated on top of
> > PTP_SYS_OFFSET_PRECISE simply by copying the sys_realtime and device
> > fields to corresponding ts slots. The apparent delay will be zero, but
> > that's ok if the conversion is really accurate.
> 
> Yes, for 1 sample only.  Otherwise you'd have the same issue as in
> Marcelo's driver (the device aka guest timestamp from
> PTP_SYS_OFFSET_PRECISE would not be halfway between the system aka host
> timestamps), and your idea below could be applied.
> 
> > I'm not sure if trying to do that in the opposite direction is a good
> > idea. An application using PTP_SYS_OFFSET_PRECISE may assume the
> > conversion is accurate and not include any delay/dispersion in an
> > estimate of the maximum error, which is needed in NTP for instance.
> > 
> > If we know the host timestamp ts[1] is not in the middle between the
> > guests timestamps ts[0] and ts[2], but rather closer to ts[2], why not
> > simply shift ts[1] by (ts[2]-ts[0])/2 ?



> 
> Interesting idea!  For this to work, KVM needs to implement
> getcrosstimestamp and ptp_chardev.c can then add an alternative
> implementation of PTP_SYS_OFFSET, based on precise cross timestamps.
> 
> Something like
> 
>                 for (i = 0; i <= sysoff->n_samples; i++) {
> 			// ... call getcrosststamp ...
> 			sysns = ktime_to_ns(xtstamp.sys_realtime);
> 			if (i > 0) {
> 				devns = ktime_to_ns(xtstamp.device);
> 				devns -= (sysns - prev_sysns) / 2;
> 				devts = ns_to_timespec(devns);
> 				pct->sec = devts.tv_sec;
> 				pct->nsec = devts.tv_nsec;
> 	                        pct++;
> 			}
> 			systs = ns_to_timespec(sysns);
>                         pct->sec = ts.tv_sec;
>                         pct->nsec = ts.tv_nsec;
>                         pct++;
> 			prev_sysns = sysns;
>                 }
> 
> Marcelo, can you give it a try?

Can convert fine, but problem is the simultaneous read
of host and guest clocks.

> Thanks,
> 
> Paolo

It seems to me anything else other than using a single TSC read
(for both host and guest clocks) is a poor PTP_SYS_OFFSET_PRECISE 
implementation (because it would claim to be similar to ART, where
the timestamps are simultaneous), but not be.