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? Thanks, Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html