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

Marcelo Tosatti
2017-01-20T10:34:43Z

On Wed, Jan 18, 2017 at 04:45:07PM +0100, Paolo Bonzini wrote:
>
>
> On 18/01/2017 15:50, Marcelo Tosatti wrote:
> >> 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.
>
> Could the TSC from the hypercall be applied to kvmclock to do this? My
> understanding is that get_device_system_crosststamp (which is used in
> the sole in-tree implementation of getcrosststamp) already contains all
> the logic to do that.
>
> Paolo

Yeah feed that TSC to pvclock_clocksource_read.

Cool will take a look at that function thanks.